That doesn't work unless you forbid proxies and vpn endpoints. And particularly with the latter, that is very hard to do unless you like to play digital whack-a-mole.
Not sure what you mean, The embedded image, or my signature? Both are pretty much easy to do. Like if I post an image in this thread I will get everyone's IP, but I won't be able to know who is who. But in a PM I would know since I only sent it to one person. I did this on another forum, once, I don't really recall the context, I was a kid and thought I was a l33t hax0r for doing it.

As primitive as it is it is a decent trick if you want to know someone's IP for some reason.
Something like Privacy Badger would probably block the hidden image though, so not sure how well that trick would work now days with everyone running various ad blockers, privacy extensions etc.
The sig thing is a bit harder to "block" as there's not really anything special being done client side. Whoever is viewing the image will see their own IP, just the same way going to ipchicken.com will show your IP, except it's just doing it with an image.
I have been asked on various forums to disable that sig because it freaked people out though, I always get a chuckle at that.