I always recommend to my blogger clients to include rich high-resolution images to their blogs. This helps to break up the text, and adds color to your blog articles.
High resolution images don't come cheap. Some people are often compelled to borrow them from other sites, but taking images without permission is never a good thing.
Istockphoto is a great source for all kinds of images, but of course you have to pay for them. The images are great, but it's easy to burn up credits and dollars using their images.
A possible source for free images is PicApp. I just discovered this site today and it seems to have a pretty big selection of images at a great price, FREE! For all of you bloggers out there not adding vivid images to your blog, check out PicApp.