Hi Gram,
I'll try to do this in order.
Advertising: I figure you need to do this. Why should the continued upkeep and hosting cost come out of your pocket. Maybe even some adsense would work.
Picture Gallery: There are some very good open source photo galleries out there that allow members to create accounts and upload photos to them. I think you would want to get some revenue in from advertising first to cover any additional bandwidth costs that may come up, or charge a small fee to cover the costs. I use yappa-ng myself and have been happy with it so far http://www.thommeyer.net/gallery/ is the link to my gallery. I have it set so I am the only one who can upload photos and create accounts but I believe it can be set so users can create their own accounts. Another good one is coppermine http://coppermine.sourceforge.net/
New Forum: I have used both v-bulletin and simple machine in the past and they are both good forum scripts, the question I have as I don't remember off hand is are they compatable with mambo or are you planning to scrape mambo and go with something different.
I think you have done a good job with this website so far and am sure you will continue to do so. If you decide to make your changes and would like some volenteer help give me a yell, I know your computer skills probably out wieght mine but I would be happy to give any type of asistance you need.