29 Apr 14
13 Jun 14 4:19 pm
The difference is between "royalty free" and "free".
The default payment for works of art like photos is royalty payments, which means you pay for every use of the photo, every printing of the book, or every play of the song. Royalty free means you pay a one-time fee and you can use the photo or other work of art any way you wish, within the terms of the agreement, which of course you do have to check.
Free images that you don't have to pay for at all are harder to find and may be lower quality. Also, in most cases, sites that offer free images require you to include an attribution to the photographer and/or website in their terms and conditions.
Personally I like crediting the creators of images, programs, and tools I use publicly, and often to do it even when I don't need to. The creators have obviously done work I like, or I wouldn't be using it, so I like to promote and endorse them to others.