There are lots of places online where you can find clipart and images to use in your classroom for those DIY projects - worksheets, center signs, stuff for themes, and so on. A few pictures can go a long way to make classrooms and classroom materials more interesting for your students. These are a few of my favorite places to find images, along with some tips for searching out and using them.
Discovery School Clip Art
Awesome Clipart for Educators
The Teacher's Guide Clipart
Oxford Science Encyclopedia
Of course, you don't have to go to sites specific to education. You can also use Yahoo! Images, Pixsy, or the excellent Picsearch to find what you need. Google Advanced Image Search is also convenient, with various filters for different types of images. Try altering your search to find only black and white or grayscale images, which are great for adding to worksheets that will be photocopied. Specify large black and white for coloring-page-type pictures on almost any subject.
Wikipedia has many images related to common curriculum topics. For photos, go with Flickr. For odd/humorous Photoshopped photos, try Worth1000. You can find fully digital images at Renderosity. Public domain images can be found at Gimp-Savvy, Moka, and Public Domain Pictures.
Bonus link: easily resize and alter your found images with pic resize.
Whenever using images you find online, be sure to check for any copyright laws that may apply. I doubt there are any realistically enforceable laws preventing a teacher from printing out a copy of an image and using it for educational purposes in the classroom. It never hurts to check, though. And I advise avoiding the use of images from TV shows and movies - companies like Disney and Viacom are notoriously selfish about allowing any unauthorized use of their characters. (Whatever. We didn't want that commercialized junk in our classrooms anyway, so there.)
Anyway, where do you find good images to use with your students? C'mon, people, this isn't a one-woman show! I know I have readers; I've got the stats to prove it. So start commenting, or else! :)