It's important that you don't "steal" images off the internet for your own use. That's really the same thing as stealing someone's article and posting it on your site as your own. In order to legally add photos on your blog, website or any marketing piece, you need to use royalty free photos.
There are many options, some which cost money and some which don't. I've listed the ones I use for you here:
Dreamstime - This site has an entire free section! You can search by keyword or category. They will show you free photos relating to your search, and then if your search doesn't find many free options, they will give you options of photos you can purchase.
Freedigitalphotos - This site offers tons of high quality free photos that can also be searched by keyword or category.
istockphoto - The photos on this site are not free, but if you don't find what you are looking for on the websites listed above, you will certainly find it here.