If a large image is posted or pasted within a user's signature, I believe there's a restriction already set in place for that when putting it in the signature, as well as a restriction on signature length. If not, there's always the requirement that all large images should be in spoilers, as there have been experiences with mobile devices where this either lags or makes the forum a bit funky.
I believe this question was asked before, and if I remember correctly, there was an answer about how HTML wasn't going to be re-enabled due to a threat previously. If you want more information, I'd contact Kaptain Ava [you can find her username in the staff widget] since she knows a bit more about it than I do.