Boy, this is an area of back and white and all shades of grey....
I too, am a graphic designer as well as a photographer. Most illustrators use "scrap" files - photos and illustrations to draw from. Whether this is an expression on someone's face, or body pose, colors, etc. Mostly this is used for inspiration and the art is used for accuracy. That is a fairly allowable use.
When someone traces over something, then it violates copyright. ie you can't just photoshop something, or make a silhouette illustration over the top of a photo without paying usage. Nor can you redraw it too accurately so that it is the major basis for the illustration.
However, obviously therein lies the rub, there is a lot of interpretation. And artists and photographers have copied ideas and approaches for eons now. It's a part of the creative process.
There lots more on this subject at
http://creativepro.com and other sites.
Jack
As far as the $1 stock sites, I buy lots of stuff from iStockPhoto, its a very competitive business and the photos work for many clients. Getty Images is up for sale and the stock photo business model has been thrown out the door and is being reinvented. Would I sell my photos through them? Probably not. But that's the subject of another discussion...