Depends on the photo, really. Often the easiest way to find out whether or not a photo is protected is to check the very bottom of the webpage where you found it. It may say something like “All content property of…” or “Copyright… All Rights Reserved.” Sites that often host artistic images like Flickr.com or DeviantArt.com will usually have the specific rules attached to that particular photo by the artist. Truthfully, people do it all the time. As long as you don’t intend to sell the image, reproduce or claim it as your own art.. you’ll usually be fine. But it is best to look for the fine print. It really depends on the individual image each time. There are different rules for different images all over the internet.