My personal first stop for article photos is Flickr Creative Commons. That doesn't mean the entire Flickr site and you should never assume that all photos on Flickr are fair game. The Creative Commons area is specifically for photos that are up to be shared. Most will require attribution, which just means giving credit. Choose the first section for that, and go searching. It can be tough slogging through all the photos, as many users are pretty liberal with their tagging. But I have found it to be the best spot for great photos on all kinds of different topics.
Another variation on the Creative Commons theme is the Wikimedia Commons. They offer a similar situation that requires you give attribution credit when you choose a photo. They have a pretty mixed bag of photos, but I have found it very limited compared to Flickr. Still, give it a shot. It all depends on your subject matter and what you're looking for.
Lastly, I'd like to recommend Stock.xchng. Most of their photos are the staged or studio style which may (or may not) be better suited for you. You can't search by permission level, so look for the little colored balls next to the photo titles. The white balls mean you can freely use the photo, and the varying shades of grey have their own restrictions. Lighter gray means you have to let the owner know, and darker gray means you have to let the owner know and give them credit. When you see the site, it will make sense.
Flickr and Wikimedia allow you to just copy any photo you find, but Stock.xchng requires a login. It's not a big deal and it means you can download the larger photo sizes.
Well, these are just 3 options for photos and they should provide you with some great illustrations for quite a while before you run out of photos.