The quality of the free sources range from not-so-good to very good with some exceptional gems. There are some illustrations among the photos offered at a few of these sites. Always read the terms of use, restrictions and/ or licenses page(s) first before using or downloading.

FreeFoto.com - free for non-commercial users
FreeImages UK - free!
FreeStockPhotos.com - free for personal or commercial use, but read the restrictions first.
ImageAfter - free for personal or commercial use.
ImageTemple - free; read the terms of use.
iStockphoto.com - free for members up to five downloads, then pay to get more; file sharing service; no commercial use.
MorgueFiles - sounds morbid, but when you go to the web site you find out where it gets its name. Free for personal or professional use. Gems can be found here.
OpenPhoto - free but read licenses first
PhotoGratis - free for personal use only
PhotoImpact Users Gallery - free for personal use only.
Photo Repository - free
Photos by Bimsan - a few photos in the growing gallery, she also offers web graphics
Picturestation - free, royalty-free photo-sharing service
Pixel Perfect Digital - free with restricted commercial use; read the terms of use 
Stock.xchng - free for personal or commercial use

If you want your quality, free, royalty-free photo source site listed here, please use the contact form to request. I will look at your site for listing consideration with no guarantees of listing on this page.

Pay - Personal and Commercial Use

The quality is as expected - very good to exceptional. Most offer one file downloading and collections on CD. I only list the ones offering royalty-free photos. Royalty-free means you pay for it once. The copyrights to the photos always remains with the photographer and/or company. Prices start from as little as $5.60/photo with a source like Corbis. Most commonly, screen-ready pics for website use cost around $30 and up. High resolution photos for print use cost more. Others require you to sign up for a paid short term or long term subscription or membership with limited or unlimited downloads. Remember to check out the Terms of Use. Almost all require registration to get comps or even view selections in light boxes.

Brand X Pictures
Comstock Photography
Corbis -go to Professional Use then select Royalty Free
DigitalVision Images
GettyImages - royalty-free and rights managed photos by PhotoDisc
Mira
PhotoDisc
Photos.com - new player
PhotoSpin Royalty-free Photos
ShutterStock - pay a fee for downloading up to 750 photos per month; one month and three month plans available
StockByte
WonderFile