I don't know why it's not working for some users. You'd need the original image they're using to figure out why it specifically is causing problems, but it's likely due to the image having weird aspect ratio like a large landscape image or really large portrait image. It's also possible whatever device they uploaded from isn't loading in the cropping behavior for some reason as again the cropping is client side, which if that fails server side just resizes it to fit within the maximum.

The sizes I recommend for the avatar are as follows.

Maximum Image Height: 160
Maximum Image Width: 160
Maximum Thumbnail Height: 80
Maximum Thumbnail Width: 80

There isn't much reason to have an avatar as big as 610px. The canvas layout won't even display an image larger than 180px.

With CB 3.x I'll be entirely rewriting all core fields including the image field, which will get a significantly more robust cropping functionality. The plan is to store the original (up to a maximum), resized, and thumbnail instead of just resized and thumbnail to allow future edits as well as minimum sizes.