Either, it's dimensions are too big, or the file size is.
115x115 pixels, and 20.000 bytes (around 19.5 KB) is the maximum size for an avatar. The standard avatars are a little larger than that, but since they are nicely done, we decided that we should keep them.

However, I might increase those image restrictions later on.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram