Custom Image Tags

Custom Image tags fire a 1x1 transparent tracking pixel — a GET request to a URL you specify. They work by loading an <img> element with your URL as the src, which causes the browser to make an HTTP GET request. The server at the other end logs that request as a hit.

They are one of the oldest tracking mechanisms on the web, predating JavaScript-based analytics. In GTM’s modern world, they are rarely the right tool — but for specific use cases involving legacy server-side logging and certain affiliate tracking systems, they still earn their place.

How Custom Image tags work

When a Custom Image tag fires, GTM programmatically creates an <img> element:

<img src="https://track.example.com/pixel?event=purchase&order_id=12345&value=199.99"
     width="1"
     height="1"
     alt=""
     style="display:none" />

The browser makes a GET request to that URL. The server receives the request, logs the parameters, and returns a 1x1 transparent GIF (or sometimes a 204 No Content response). The tracking data is in the URL parameters.

Creating a Custom Image tag

In GTM, go to Tags and click New.
Select Custom Image as the tag type.
Enter the Image URL. This is the pixel endpoint with your tracking parameters appended as query parameters. Use GTM variables in the URL: https://track.example.com/pixel?order_id={{DLV - transaction_id}}&value={{DLV - ecommerce.value}}
Enable Cache Busting unless you have a specific reason not to. With cache busting enabled, GTM appends a random number as the gtmcb parameter to prevent browser caching from suppressing the pixel request.
Configure the Cache Busting Query Parameter name if gtmcb conflicts with your endpoint’s expected parameters.
Set your trigger and save.

Tag Configuration

Pixel - Affiliate Network

Type: Custom Image
Trigger: CE - purchase
Variables: DLV - transaction_idDLV - ecommerce.valueDLV - ecommerce.currency

URL construction

The Image URL is the full pixel endpoint including query parameters. Combine static text with GTM variables:

https://tracking.affiliatenetwork.com/pixel?
  type=conversion
  &order_id={{DLV - transaction_id}}
  &revenue={{DLV - ecommerce.value}}
  &currency={{DLV - ecommerce.currency}}
  &gtm=1

In practice, this goes in the Image URL field as one line:

https://tracking.affiliatenetwork.com/pixel?type=conversion&order_id={{DLV - transaction_id}}&revenue={{DLV - ecommerce.value}}&currency={{DLV - ecommerce.currency}}&gtm=1

Cache busting: enable it

Browsers cache image responses. Without cache busting, a browser that has previously loaded your pixel URL might serve the response from cache instead of making a new request — meaning your tracking hit never reaches the server.

GTM’s cache busting option appends a random number as a query parameter (by default ?gtmcb=1234567890), making every request to a unique URL. Unique URLs are not cached.

Always enable cache busting for tracking pixels. The only reason to disable it is if your server-side endpoint has strict parameter validation that rejects unknown parameters — in which case you can set a specific cache-busting parameter name that your endpoint expects.

Limitations of Custom Image tags

Understanding why these tags are declining in use helps you know when to avoid them:

GET requests only: No POST data. Everything must fit in the URL. GA4’s standard hit URL can be several hundred characters. Long ecommerce data (many items) may exceed URL length limits (typically 8KB for a GET request).

No response handling: You cannot read the server’s response. If the pixel fails, you will not know. Contrast with fetch() in a Custom HTML tag, where you can handle errors.

Blocked by content blockers: Ad blockers and privacy-focused browsers aggressively block pixel URLs, especially those on known tracking domains. The same is true for JavaScript-based tracking, but Custom Image pixels tend to have higher block rates.

Not suitable for important conversions: For valuable conversion events (purchases, form submissions), use navigator.sendBeacon() via a Custom HTML tag — it is more reliable, works during page unload, and handles large payloads better.

HTTPS required on HTTPS pages: If your page is HTTPS and your pixel URL is HTTP, browsers will block the request as mixed content.

No consent-aware behavior: Custom Image tags load immediately when fired. They cannot check consent state. You must control consent at the trigger level — using tag sequencing with a consent setup tag, or exception triggers.

When Custom Image tags are still appropriate

Despite their limitations, Custom Image tags are the right choice for:

Legacy affiliate tracking systems: Many affiliate networks still operate primarily with pixel-based tracking. Their dashboards expect specific pixel URL patterns, and a Custom Image tag is the most direct implementation.

Server-side logging endpoints you control: If you have a simple server endpoint that logs requests (for server-side deduplication, record-keeping, or backup tracking), a pixel is efficient and lightweight.

Simple acknowledgment signals: Where you just need the server to know “this user reached this page” and you do not need response data or complex parameters.

Third-party systems with no API alternative: Some ad networks, verification services, and data providers only support pixel integration.

Modern alternatives

For most use cases where you might reach for a Custom Image tag, these alternatives are superior:

navigator.sendBeacon() (Custom HTML tag): Sends a POST request asynchronously, works reliably during page unload, supports larger payloads. The modern replacement for beacon pixels.

<script>
(function() {
  try {
    var data = {
      event: 'purchase',
      order_id: '{{DLV - transaction_id}}',
      value: {{DLV - ecommerce.value}}
    };
    navigator.sendBeacon(
      'https://track.example.com/hit',
      JSON.stringify(data)
    );
  } catch(e) {}
})();
</script>

GA4 Measurement Protocol: For conversions that are important enough to warrant server-side confirmation, use the GA4 Measurement Protocol from your server. Client-side hits can be blocked; server-side hits cannot.

Native vendor templates: Most major advertising platforms (Google Ads, Meta, LinkedIn) have GTM Community Templates that handle consent, retries, and advanced features that a raw pixel cannot.

Common mistakes

Forgetting to enable cache busting

A Custom Image tag without cache busting may never fire in practice for return visitors, because the browser returns the cached pixel without making a new network request. Enable cache busting.

Putting sensitive data in the URL

GET request URLs appear in:

Browser history
Server access logs at every proxy and CDN between the browser and your server
Referrer headers when the user navigates away

Never include email addresses, user identifiers, order details, or any PII in a pixel URL. For data that must be passed, use POST via sendBeacon() from a Custom HTML tag.

Using Custom Image tags for high-value conversions

For a purchase confirmation pixel, use a more reliable mechanism. Custom Image tags can fail silently — the pixel request can be blocked, cached, or dropped without any indication. For revenue-reporting conversions, use a method with confirmation.

Custom HTML Tags The more powerful alternative for complex pixel logic, consent checking, and POST-based tracking.

Conversion Linker The Google tag companion for Ads conversion tracking — more reliable than image pixels.

Tag Sequencing How to ensure consent is checked before your pixel fires.