If you look at my blog I have really struggled with really good (low / no cost) blog images. I came across this guys site today - it's totally devoted to great blog images and has a lot of great content on it - http://www.betterblogimages.com/the-1-blogger-friendly-website-for-finding-citing-free-images/
Another good article on the subject - http://www.incomediary.com/how-to-use-photos-in-your-blog-to-make-it-more-interesting
And my favorite on the subject I have come across in awhile - http://www.incomediary.com/how-to-make-your-blog-posts-stunningly-beautiful-images
Good luck!
Matthew
