Getting posting errors after turning on Thumbnail support.

I turned on thumbnail support for my theme so that I can use the Category Posts widget, but now whever I publish a post with a thumbnail, I get the following message.

Warning: Cannot modify header information – headers already sent by (output started at /nfs/c04/h03/mnt/64590/domains/albotas.com/html/wp-content/themes/atahualpa344/functions.php:3) in /nfs/c04/h03/mnt/64590/domains/albotas.com/html/wp-includes/pluggable.php on line 868

I think I did something wrong because I sort of just threw the thumbnail php thing somewhere in the themefunctions php.

Any help would be greatly appreciated.