I'm not the best web designer out there, but I have learned a nifty trick to assist my un-WordPress savvy clients still be able to maintain an organized look to their blog pages. I create tables for them, where they can enter information into the cells and keep everything on the page nice and tidy...just the way I originally delivered their website/blog.
So, I used this same tactic for the Gallery slideshow which had me terribly stumped on how to center it. I create a single row/single column table that has a center alignment. Then I place the Gallery code into the table, and Voila! It is perfectly centered as long as the width of the table matches the width I have set the slideshow to be. Hope this helps out!!