One also needs to bear in mind that people access E from all kinds of mobile devices and on all kinds of mobile networks. The site needs to be accessible for them as well. I think that all of E (ie text) loads after
all images do, which, on mobile network can be like forever and it's the reason why things in sigs need to be uploaded to the wiki first. There were too many instances of other hostings not responding and E not loading for users.
Personally, I think that this is a literary forum. If we were a drawing/painting/computer graphics community, I think we could definitely use a more colorful sigs. But we're not and excessive images disrupt the flow (for some people). Plus, there are so many options how to make your sig interesting with the text options available. For example, Caeli
is and has been a stellar example for me of a text sig done exactly right.
I am absolutely not saying that there shouldn't be an option for more images in sigs. I'm just saying that there already are options how to make visually appealing signatures, they just aren't being used to their fullest.