Just a stab in the dark here, but have you...
1) checked to see if there are files in the avatar directory?
2) checked the perms on those files (0666)
3) since moving from Win to Nix, dbl check the file names and be sure your case is correct, ie; FiLeNaMe.JpG will need to be just that on Nix but windows will ignore upper or lower case and treat it as the same.
Are you getting an error when trying to upload a new avatar?