*delurks*
There are a number of professional stock photo sites you can use, if you want to go the hi-res, licensing route: shutterstock, istock, getty, 500px are but a few. You'll often find price breaks if you buy bundles, rather than photo by photo. Make sure you read the attribution rights.
Some free / creative commons arenas as mentioned above: 500px has some creative commons photos that are of high quality (I suck at trying to figure this out at times, so if someone has that nailed down, I'd love to hear the secret...I'm always staring at those going "did I do the search right? is this REALLY cc license?"), Flickr does as well (check the CC license level for attribution particulars; and also they aren't supposed to be able to do take-backsees once they've set something as a creative commons license, meaning they shouldn't be able to claim it's all rights reserved later, but proving that seems to be a sticky-wicket on Flickr later), pixabay, usplash. Getty images has an embed option for bloggers for non-commercial use, but be careful you read all the terms. And embedding doesn't exactly play nice with all of Wordpress's features.
You can also use Google's advanced image search features that will include attribution information, but you should always go to the original source site to verify it IS the source site and the licensing info is correct. Better safe than sorry, folks. And don't just grab it off Google - poor quality and potential low res, among many other reasons.
Also, a few warnings I often share at my day job where I have to police this very thing in presentations our technical experts/scientists like to make:
1. Do not Google grab. Ever. Google grabs are bad, m'kay? So's a Bing grab
Also, don't randomly swipe images from other sites/blogs/tumblr, etc. You are asking for trouble. Like say, if you swipe a professional Getty image? Woooo mama, that could be an EXPENSIVE mistake. Because they will hunt you down. (Seriously, not kidding. Getty is known for that.) And hey, as a writer, you wouldn't like it if someone just swiped your story or words and posted it on their site as their own, right?
2. Make sure you understand the attribution rules if you are using a photo under Creative Commons. And if you don't like the particulars that the owner of said photo is asking for in the credits, then don't use the photo.
3. All rights reserved means hands off, Bub. Unless you desperately want THAT photo or THAT digital work of art, then ask for permission. In writing. This is called cover your butt.
4. When in doubt, don't use it. Find something else.
5. Public domain means you don't have to give credit, but hey, it's kinda nice, so if it makes sense...why not give the owner some love?
Using your own art/photos is always the easiest, but also bear in mind that once you put it out there...remember rule number one above? M'yeah. Not everybody follows that. So give that a little thought before you sling your photos and art out there, too.
*goes back to lurking*
