I'm not sure if this is a bug of the text-ui or of adium themes (tried GoneDark, and a few others) but if avatar height and width are different the avatar is scaled unproportionally to fit exactly in the square area
That is theme dependant. I fixed it in renkoo (downstream). I can't change things that are on adiumxtras.org. The only thing we could possibly do is take the avatar pad it to a square and save it out again, but that's a massive overkill to work round someone else's bug. P.S adium themes all suck anyway.