How do I use custom image tag?

Question

I am just starting to learn GTM, and I feel like this question is a doorway through which I can better understand how one should use GTM.

Previously I've been using GTM to insert GA code to my site, so I am not completely unfamiliar with GTM. But I don't understand what custom image tag does, there are several questions:

1) What is it used for? In Google Help Documents, there is only a small passage regarding custom image tag, https://support.google.com/tagmanager/answer/6107167?hl=en, where 'track' and 'pixel image' are mentioned. I wonder whether custom Image Tag is just used for tracking website data?

2) What are those options for image URL for? Intuitively, I'd think image URL is where you input your custom image's URL. However, here I can choose "referer","custom event" etc., these are totally different from URL, so what are these for?

3) Once the tag is set, where to see its outcome? For GA code, it is straightforward, you just go to GA page to see the outcome of your Tag. But for this custom image tag, although I've set it up, I don't know where to watch how it works, and the google help doc is not very helpful.

Answer 1

1) In most cases this tag is used to get that tiny image pixel for tracking purposes. Other reasons for using this tag are quite complicated and not popular.

2) Image tag is used only for getting a custom image. But you can get the URL of this image from wherever you want. This includes event, referrer etc. This is the default option for such fields in GTM.

3) You can look through the log files of the server, from which custom image is fetched. You will see some requests there. If you try to construct measurement protocol request (https://developers.google.com/analytics/devguides/collection/protocol/v1/) through custom image tag then you will see this request in real time reports in Google Analytics.