Yeah. it's the strangest thing. I've had similar problems in the past. Even when you try to link up an avatar within size (which you can easily verify), it wouldn't go through.
What I had to do was, change my avatar to "someone elses avatar" that is already on this forum.. And then rechange that to the one you want. Sounds crazy right?.. Well it worked for me consistently like 3 times, when I retried the process.
The darn the bothered me so I went back to the default and tried to change again, but didn't go through. But when I used that intermediate step, it worked.
Who know why. But who cares