This is a great question. The rule of thumb is to always get the highest resolution which is available. The higher the resolution, the more options you have when you are printing a photo. It's also important to know what size you are planning to print the photo at: the larger photos require a higher resolution, otherwise you risk the photo looking pixellated.
Which Stock Photo Agency for Print Photos?
Some of the major sites which offer stock photography are iStockphoto, Dreamstime, Shutterstock and Fotolia. For example, most of these sites provide a 72 DPI and 300 DPI resolution option. If you don't know what DPI means – we explain DPI here. For maximum flexibility, pick the size closest to what you are printing, and the highest resolution possible.
Do you want to resell the prints?
Also remember that if you are buying photos to resell or for a client, you will need an extended license to cover the reuse – always check what you can, and cannot do with a photo based on the type of license selected, before you purchase it.