the real issue lies in the forum's bad attempts to detect uploading of malicious headers in image files
it detects a lot of false positives in popular programs like Paint.Net etc.
don't listen to people who tell you that your avatar needs to be under single digit kilobytes or need to be a specific size
try bringing your avatar into a different image editing program and saving it with that program as most image editors have slightly different ways of saving the headers
also keep in mind after uploading an avatar they all go to the same url so it will take a little while for your browser's cache to update the avatar to the new one or you can clear your cache manually, eventually everyone will see your updated avatar and not the old one