poprockgeek suggested I post this link here, so here it is
This site has hundreds of Doctor Who images, and is constantly adding new ones. They only cover since 2005 (so no 'classic' Who, I'm afraid), but they do have images of:
In-Episode Screenshots
Behind-the-Scenes (on-set) Photographs
Promotional Pictures
Desktop Backgrounds
IM/Forum Avatars
Posters
Concept Art
WARNING! Some images on this site may cover unaired episodes. If you don't want any spoilers, you should be careful.
There are 2 ways of accessing the site.
The first is the Russian website. If you are able to translate it (I use the Chrome browser which autotranslates for me), then you might find this easier.
http://photos.telestrekoza.com/British_shows/Doctor_WhoThe second is the 'back door', and accesses the actual files on the server in a folder tree. This method won't give you any thumbnails so the only way to see what each picture is, is to open it. The folder tree is in English though, so is easy enough to navigate.
http://photos.telestrekoza.com/var/albums/British_shows/Doctor_Who
