I am illustrating a book and I have used some stock art mixed with design in Illustrator and Photoshop along with a few hand drawn pieces. Can I say Illustrated by "my name" if I do use some stock art?
Read the license associated with any stock images.
In most cases, you do not own the copyright, even if you do alter the image some.. and you can not claim the artwork as yours. But again, this does depend upon the actual license granted for any stock images.
You could use a couple variations on credits....
Book design: Helen XXXX. Contributing Illustrators: iStuckProto, Salvador Dali, Ed Hardy, Keith Herring, and Pablo Picasso.
You would then list the artists of the stock images you used. If you created original artwork, you can list your name there as well.
Original Illustrations by Helen XXXXX
This indicates you are only responsible for the original artwork and kind of skirts the issue of stock images neither claiming nor denying ownership directly. It's kind of iffy morally. I always prefer the first option I provided above. It correctly credits the contributors.
Again.. altering a stock image does not make the artwork yours.
No, you can't put in artwork from other people in your design and pass it on as yours. Everyone is guilty of copying/using elements from other people's design but you can't sell your work with other people's art in it.