Why? Because I don't want to be sued for a picture that someone has the rights to. So, I stopped using pictures on my blog, but recently I found Flickr. Flickr is a photo website with a large gallery of images. If you search something like "drumset," using the advanced search tool with Creative Commons check boxes checked off, you are allowed to use what you find on a blog, in your book, book trailer, etc.
I hope this helps, I know it did for me!