Of course, you always have the option of taking a photo yourself and uploading it if you want to avoid problems, but if you're not good at photography, the results can be less than stellar. Three places I have obtained images for my work for clients include freedigitalphotos.net, morguefile.com, and Flickr's Creative Commons collection.
I use freedigitalphotos.net the most, because they have the biggest selection of high quality images (including drawings), and they're free to use as long as you provide proper attribution. If you (or your client) doesn't want to include attribution, their prices are reasonable enough that you (or your client) can buy them and skip attribution.
Morguefile.com can be a good source, but the quality of images varies widely. Still, it's worth a look. I don't use Flickr's Creative Commons collection very much, but it's good to know it's an option. One thing I found when searching these images is that some of the ones that come up in a Creative Commons search are actually not licensed under Creative Commons. You can get into legal trouble if you accidentally publish something as Creative Commons when it's actually under a standard copyright, so read terms under individual images carefully.
To keep my image library organized, I download legal images into a folder on a flash drive, and I have a spreadsheet listing the link to the photo, the filename I give it, and the attribution. This makes it easier to reuse images, and cuts down on time spent trying to track down the perfect image online.
Images can add interest to your personal blog, and clients will ask you to provide them sometimes, because they help draw traffic and break up walls of text. Be prepared, organize the images you download, and keep it legal, and before long you'll have a nice image library you can turn to for many of your image needs.
Photo Credit: Stuart Miles / freedigitalphotos.net