And just to advertise myself, if you wanted to see the Ringo Starr drawing, here's a link to my drawings on flickr.
Step 1: Requirements
Step 2: Glue Tacks
Step 3: Glue Magnet on Poster
Step 4: Hang
My poster aren't perfectly tight against the wall. There's a very slight pooch in the middle. Other than that I can reposition my posters freely without worrying about damaging them. If you want to post 4x6 photos, you may even be able to just glue on the smaller magnets and use regular thumb tacks without magnets.
I used VIVYX printing to print my posters and purchased the tacks from 1-2-3 Stitch . If you would like to view my drawings, they're on flickr.