What are the dimensions of your image? ISTR there is a limit on the size which is why mine is only 70x100 pixels ( but I couldn't see the limit when I just looked at the page where you set it)
The Avatar image I have now (Synthesis Web) is 1440X1080 and 296 KB. When I tried to post a 2560X1920 and 2.46MB it prompted the error message below. I could not find any information about the size limit of your avatar.
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