Could avatar requirements be little relaxed? 6kb limit for 90x90 it is too little. I can't really pack anything into it. Just having 90x90 it is 8100 pixels, if each coded with 3 bytes, will be 23kb, and about 15kb with compression.
I was forced to convert the image to 256-colors indexed palette (1 byte per pixel), remembered about such a color model first time for the last 20 years, and then do 5 attempts to JPEGging it, balancing between the remaining quality and 6kb limit. And I don't like his face color now
Probably, for most of users that will be impossible task.
If it is possible, please raise boundaries a bit (say, 120px) and limit for 40-60kb.