Why...

Why aren't images allowed in sigs?

They would probably take up too much space and everything. You will have to wait until one of the moderators posts here though, because I obviously can't answer for them.

Ok thanks! But they could probably just make rules to stop from making them too big..

There's a number of reasons:

It's against the site rules.
It's esthetically un-pleasing when it's widespread.
It makes pages load very slowly.
We're a discussion forum, not an fanart forum.

Most people tune out sig lines anyway.

Aww ok. I love making fanart. I guess there's IDF for that. ^_^