Yes, netPBM and ImageMagick as well as many other utilities are installed on all servers.
As for Gallery2, this is available as a 'one-click install' from the web admin panel.
All the domains and sub-domains you host at DreamHost are, for the most part, equal. There is no real 'main domain'. Each domain (or sub-domain) can be configured independently and each has its own web accessable directory.
As for using your own DNS servers; This could present a few problems as the IP address of your particular DreamHost server may change without notice, or your site may be moved to a completely different server (and IP). Of-course, this problem can be overcome if you purchase a unique IP address for your domain.
I am not sure. The best option would be to contact DreamHost and ask them.
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