You may have to use a laptop or a pc. Wikia won't allow certain mobile devices to upload avatar pics. I use an iPad and was having the same issue, so I asked Knj00 shortly after signing up here. It may be all mobile devices or just certain ones, I can't remember. And it's kinda weird, because you can use the same device to upload images to posts and other areas of the site, just not avatars. I still haven’t bothered firing up the ol' laptop to do mine yet.😄
I hope this helps and you can get it figured out!
Using the full site will not fix this issue. According to the Wikia help pages, it's a problem with JPEG image files and mobile devices, particularly iOS.
(See note under 'Avatar Location' heading)