Re: cropping doesn’t work well…
Yes, that does make sense.
If you look at bp-core-avatars.php, it sets the small avatar version at 50 by 50 and the large at 150 by 150. So, if your source image is smaller than 150 on at least one of its dimensions, you could have issues with the large avatar creation.
Do you see the black bar only on the large avatar version?