I'm afraid that's actually a bit of a misconception. Some images and some image stock sites have restrictions on what you can do with the photo/image you've purchased, how many times you can use it, if you can store it or not, and on what media you're allowed to use it on.
I'm a member of many design forums, and a common occurrence is people being sued for using images incorrectly. I know it sounds like overkill but I'd suggest you give the T+C's a glance before you purchase an image.
Just whatever you do- don't be tempted to just use Google images and take images from there. Every single one of them will be copyright protected.
I generally tell people to use only use images that
1. You've taken yourself
2. You've purchase and read the T+C's
3. A supplier has given to you and you have a written/email confirmation that you can use them for the project you intend to use them on.
It's a bit if a minefield to be honest, but as Izzydoll says above, search for "the getty letter" for the reason you have to be VERY careful.
