You could....
..Have one about the actual city, like a layout of the city with smaller pictures of important rooms in the city.
..Have one about the writers, directors, producers, hairdressers, et cetera AKA the crew, the people who actually keep the show running.
..Have one about the actors and actresses, and the controversies and fan responses over their hiring or firing (good ones for that would be Ford, Weir, Carter, Keller) (this would be the one time to be happy that the producers are always changing the cast).
..About important technology on Atlantis. Stunners, puddlejumpers, GDOs, satellites, darts, hiveships, Daedalus, etc. That would be interesting.
..One about Beckett. Yes, all about Beckett :) There was a huge, positive, fan response to this minor character when he was introduced, which helped the role grow larger and larger. And fans were incredibly angry when he was killed (look it up, there were petitions, wee baby turtles being shipped through the mail, and even more crazy things happening, things I probably shouldn't talk about. But the fans were so very angry). Also, think about the way they brought him back (it was supposedly "planned" all along, which seems mean), it's actually a clone not the original. Of course, the whole Beckett thing is a bit long so if you want to skip the whole thing, it might be wise (or just make two or three slides out of the Beckett story). If you do use it, look in The Stargate Atlantis Official Companion To Season Three by Sharon Gosling, that book has a lot of info about the actor, the fan response to the character death, etc.
..Consider having three slides for characters. Good guys (at least originally), then bad guys, then minor characters. Have a little paragraph to go with each picture.
..Make sure you include in the actual Stargate slide the answer to the question why the Atlantis Expedition normally has to use the Daedalus to get back to Earth (it takes too much power from their precious ZPM if they go via the Stargate, though it would be quicker).
..Have one on the other important or recurring alien species besides the Asuran, Ancients, and Wraith. For instance, the Athosians, the Genii, the Hoffan, the Travelers, (a group, not a species) The Brotherhood of the Fifteen, the group of kids in Childhood's End, et cetera.
..Have one about the ZPM. It's a huge part of the show in its own quiet, bright orange way.
..Have one about "spoilers", upcoming events.
..You could always ask Joseph Mallozzi what he thinks you should do or what he thinks of your slide program. He's one of the producers/writers on the show and he has a blog where you can ask him questions. Link: http://josephmallozzi.wordpress.com/ (I haven't checked on the blog for awhile now, because of school, so I hope he still has the Questions and Answers part, it's called Mailbag, I think).
Hope this helped!
If this isn't as helpful as you hoped it would be leave a comment explaining what else you need and I'll edit my answer. Good luck!
I added more stuff to my answer so I hope it helps. Also some links that might help are:
http://en.wikipedia.org/wiki/Stargate_Atlantis
http://en.wikipedia.org/wiki/Stargate
http://en.wikipedia.org/wiki/Stargate_%28device%29
http://en.wikipedia.org/wiki/List_of_stargate_atlantis_episodes
http://en.wikipedia.org/wiki/Human_civilizations_in_Stargate_Atlantis
http://en.wikipedia.org/wiki/Alien_races_in_the_Stargate_universe#Pegasus_Galaxy_aliens
http://en.wikipedia.org/wiki/List_of_Stargate_Atlantis_cast_members
http://en.wikipedia.org/wiki/Stargate_literature
http://en.wikipedia.org/wiki/Fictional_materials_in_the_Stargate_universe#Wraith_enzyme
http://en.wikipedia.org/wiki/Atlantis_%28Stargate%29
http://en.wikipedia.org/wiki/Zero_Point_Module
http://en.wikipedia.org/wiki/Category:Stargate_technology
All of these links should help. If you aren't allowed to list Wikipedia as a site then these websites (listed below) will have the exact same information and more:
http://stargate.wikia.com/wiki/Stargate_Wiki
http://www.gateworld.net/
http://www.stargate-sg1-solutions.com/index.php
Did you ask Mr.Mallozzi for help? I'm sure he would help.