Royalty-Free Stock Photography Sources
When you want to use photos on a website (on in other media like print and television), you generally must have a license that gives you permission to use the photo in a certain way. You could hire a photographer and models and obtain releases from them yourself, but this would be very expensive for a small business.
That is why companies turn to royalty-free stock photography to get professional looking photos for a relatively small amount. Royalty-free means you generally get a license to use an image in an unlimited amount of ways for a single set license fee -- and you are free from having to pay a royalty everytime you use an image.
When using stock photography websites, you should always look for the "royalty-free" photos NOT the "rights managed" photos. With rights managed photos you must obtain and negotiate a specific license for a specific use. While rights managed photos may give you some exclusivity over the use of an image, they also are generally VERY expensive. That is why you should always look at the royalty-free photos when building a website.
Here are some good sources for "royalty-free" stock photography - they generally vary somewhat in price, and some are subscription based:
7) Getty Images
Note that the first 6 sources listed above are generally considered "microstock" photography sites, where photographers generally charge $1 to $20 per photo. Getty Images (which actually owns iStockphoto) on the otherhand, generally charges far more to license their photos - from $50 to $300 for most photos.
When purchasing photos, size is always a consideration. Generally for display on the web the "Medium" size photo version will suffice -- although the sizes vary from site to site and you should look at a sample to make sure you are ordering the right size. For print needs, you generally would need the larger versions since a higher resolution is necessary to make photos look nice when printing.