The common practice of grabbing the first image off of Google Image Search is not the way to get a photo for your blog post. These images are often copyrighted and should not be used for any other purpose than their original, intended use. To find free, fair-use photography for that featured image on your blog post, go to these alternate (and free!) sources.
KEEP IN MIND: While all of these sources provide free, high-resolution images, some images require you to contact the photographer to simply mention what it is being used for, and where it is being used. If you see a user agreement, do take note of it.
1. Morguefile: Great site with plenty of high-resolution images. Does not require an account - you simply click on the "Download" button, and you'll have the photo.
2. stock.xchng: Stock.xhcng has the largest selection of images than any other free stock-photography website. Does require an account to download, which is free. They do show you paid search results as well, so be sure you are not selecting from the "Premium" area, or you will be bumped off to one of their partner sites.
3. Image*After: No account required (which is convenient), but more useful for textures than specific images. For example, a search for "college" recently brought up only 2 images, and one was obviously eastern european.
4. Wikimedia Commons: The majority of images on Wikipedia are free-use, and Wikimedia Commons is a great place to search these. When you click on an image, it will let you know the license agreement and whether it is copyrighted (not many are, but some will be) or whether it is public domain.
5. FreeRange: A site with some very high-quality images, you will have to create a free account to access and download them. Certainly a place to check out.
6. Google Image Search: Google can be used to find free-use photography, but you have to know how to use it correctly. First, you must click "Advanced Image Search" to the right of the search field. Now, fill in the search field with the topic you are searching for. Finally, scroll down to the bottom of the page to see the "Usage Rights" section. Once you have found it, select "Only images labeled for reuse." Search!