I've used import image plane a few times but since I haven't done much experimenting with that I just create a poly plane to match the pictures resolution. For example, if a picture has a 640x480 resolution then I'll create a poly plane that is either 6.4 x 4.8 or one that is 64 x 48, or even 32 x 24 (anything with the same ratio) Then, after I attach the picture, I do a planner map to make sure it's not stretched or anything. Works really great for me. I put each poly plane on a different layer so I can turn them on and off individually.