The photo gallery and forums are two different peices of software that don't talk to each other. So your forum username and password won't work on the gallery.
If you want to email me with a username and password (can be the same or different than the forum. Doesn't matter) then I can set you up to add or delete photos from your gallery.
I have plans to revamp the whole forum and gallery system with vBulletin as soon as their latest release gets out of beta. Then the gallery and forums will be totally integrated. (Which is nice for all of you, but even nicer for me since I don't have to do it all manually)