In some scenarios, an agency, advertiser, or other third party might decide to track impressions with a tracking pixel. A tracking pixel is simply code inserted into a custom or third-party creative that makes a server call and returns a transparent 1×1 image (normally a GIF file).
Sources of third-party tracking pixels
You typically get a third-party tracking pixel from a third party, in the form of a URL. You then insert the URL into the tracking pixel code and add the tracking pixel code to the Code snippet box in the third-party creative page.
Most tracking pixels have the same format with slight variations in the style variable. It is up to you and the advertiser to choose which format to use. Here are a few examples:
<img src="TRACKING-PIXEL-URL-GOES-HERE" style="position:absolute; visibility:hidden"> <img src="TRACKING-PIXEL-URL-GOES-HERE" style="display:none"> <img src="TRACKING-PIXEL-URL-GOES-HERE" width="0" height="0">
Add tracking pixels to creative code
If you want to add a tracking pixel to the creative code of a custom or third-party creative, simply insert the tracking pixel code at the top of creative code in the Code snippet box.
Make sure there is a hard return between the pixel code and the rest of the creative code.
<img src="TRACKING-PIXEL-URL-GOES-HERE" width="0" height="0"> <!-- Template ID = 4439 Template Name = Image Banner - Open in New Window--> <a href="%c%u" target="_blank"> <img width="728" height="90" border="0" src="%h/2315223/728x90Image.JPG"> </a> Source code taken from https://support.google.com/dfp_premium/answer/1347585?hl=en