You can't just go to Google Images and plunder images without permission or due acknowledgement folks. It's illegal.
So how to get around this? Flickr is one notably good option. Now, when I use pictures I'm sure to acknowledge my source. But I don't have to for this one:
Ah what a pretty picture. It's High Force waterfall in County Durham in case you were wondering. Why haven't I had to acknowledge it? Because I took it, duh. It's on my Flickr photostream, so I can use it no questions asked. If you haven't got a Flickr account, you might want to consider setting one up.
If you want to use the above photo, or indeed any photos on my photostream, then you need to say where you got it from. BUT only if the person in question has put the photo under a Creative Commons licence, which means they allow you to use their work. They'll normally ask for you to give them credit for the photo, which is fair enough. So this I how I do it:
Photo courtesy of schammond
SC Hammond is a friend of mine in both Flickr and real life, and I chose this because she and I have the same favourite book: Catch-22. It's up to you how you acknowledge your Flickr source, of course. But giving a link that will take you to the relevant person's Flickr account is always good in a you-scratch-my-blogging-back-and-I'll-scratch-yours-way.
When it comes to book covers, which can be harder to find on Flickr, be sure to provide a link to the author's website (where you should be able to source the image from), or it's Goodreads profile. Once again, as well as ensuring legality, the author will be grateful for the traffic you direct to their website. Even if you trash their book...maybe.
Oh and if you want to use my photos from my Flickr photostream go right ahead. Just be sure I've given you the permission to do so!
(Yes, this is one of mine)