I'll go down the lines.
There are many sources of images, but what you're looking for is called stock photography. Stock photography is a picture from a pre-collected set of images taken by professional photographers. They are pictures that are usually for sale that can be used in many situations.
The creative commons license means (and this is in general - make sure to view the specific license before you start, done by clicking 'creative commons license') that the image is free to be used for personal or commercial use. Usually, you have to link back to the original photographer or provide some sort of credit.
Royalty free are a type of paid images that have a single, one-time fee. These differ from Rights Managed photographs that have continuous fees over time, depending on an assortment of factors (how often it's used, how much profit you make from it, etc.).
Under the Creative Commons license, you usually have to give the photographers name. This can be as easy as putting a 'sources' or 'credits' link in the footer or putting the photographers name in the 'alt' attribute of the photograph. For instance, you could do something like this:
<img src="http://farm3.staticflickr.com/2162/2341162923_3c492c670c_z.jpg?zz=1" alt="Fly in the rain by Nam Nguyen" />
Edit: I think that that's XHTML, but I can't be sure w/out checking. So confusing these days!
So if you hover over the image, you'll see the name (if no other effects are in place). Easy as that, and very unobtrusive. If you contact the photographer, he might let you use it without reference, but I highly doubt it. As in, very highly doubt it. His specific policy states that you need to reference him if you use it, but it's OK for commercial work.