I'm working on a poster for a local event. It has a photograph in the background. The poster itself is designed to be 11x17, which at 300dpi is about 3300x5100.
Now the photo in the background does not have to be completely crisp, since it is behind the text and can be a little blurry.
I'm just looking at istockphoto and wondering what size image to buy. I don't think I need the full size, but I have no idea which I can get away with.