Unable to View Thumbnails Despite Full Permissions [SOLVED]

I have a local installation of Typolight on my development computer, and I can't view thumbnails. Image galleries and the file manager always display images at full size. I've given both the system/tmp and system/html directories full rwx permissions of 777.

I've also purged the thumbnail data. Is there anything else, besides file permissions, that could be confounding my ability to view thumbnails?

Thanks

Maybe you don't have gd2 installed?

That's it! Thanks. I was using the default Ubuntu package for php5 and it doesn't come with GD while the remote server does.