well first...i went to thecaconline.net/cbl
and i went to search my name..and when you do that..you get all your stats and a picture of you on it...your ingame picture..i just copy and pasted it on my desktop and uploaded it to the website as my avatar...
you said you wanted to get a merc avatar...so first you would need to be able to get yourself a merc..and do the same thing...or when you go get a picture from thecaconline.net/cbl, make sure the picture you get is a picture of a merc that you want..which could be mei or baron or whatever the merc's name is
hope i helped, or do you still need some help?