Share 'Images to Use for my blog?'

I would like to make a header for my blog to make it more visually appealing. I want to use some of my favorite authors' books but I'm afraid to violate one or more of the hundreds of rules out there. Can someone tell me if I can use book covers or if not, where I can find free images (stock) that might pass as covers to romance novels? I doubt I'm the only person who'd like to know the answers t…