If you get an error saying

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

when you try to put a new custom header image for WordPress theme 2010 on a server using Ubuntu the following may help:

[bash]apt-get install php5-gd[/bash]
once it installs then
[code]invoke-rc.d apache2 restart[/code]

This will provide php the ability to manipulate images that WordPress is trying to use.