One of the best SEO strategies for a blog is including legal to use pictures and photos, allowing you to also capture traffic from picture search engines as well as search engines for text.  The biggest question is, what pictures are legal to post on a blog and which are not?

The legality of a picture or photo is usually a very grey area that can get some bloggers into trouble.

1. If a photo has a copyright, don’t use it.
2. If the photo is from a website in which you need to pay for pictures, don’t use it.
3. Any photos of high profile people should be used with caution.

Source to Find Legal Pictures, Photo for a Blog
I often use Flickr as a source for finding blog photos.  Photos will be marked whether they are copyright, and if they are you won’t be able to right click on the photo, so there is already a system in place to prevent you from accidently using a copyrighted photo.

Try using Zooomr as another method to find creative commons photos. You might also try  everystockphoto or stock.xchng.