Im Getting Error in Croping Images

Im receiving this error when i upload image in User->Your Avatar

1.) Upload
Image could not be processed. Please go back and try again.

2.) Alternative Upload
There was an error uploading the file, please try again.There was an error uploading the file, please try again.
Warning: Cannot modify header information - headers already sent by (output started at /home/rfnet/public_html/wp-includes/l10n.php:189) in /home/rfnet/public_html/wp-includes/pluggable.php on line 899

Same things Goes Thru Setting---> Blog Avatar

1.) Upload
Image could not be processed. Please go back and try again.

2.) Alternative Upload
Fatal error: Call to undefined function ImageCreateFrompng() in /home/rfnet/public_html/wp-content/plugins/avatars/avatars.php on line 576

I need Help Fixing this Stuff, I Dont Need the BBpress Avatar Only the Multisite Avatar but its not Working, And Cannot Crop Images...

A help on this Will be Appreciated