I just finished a moderately ambitious video and all the images came from the free stock photography site stock.xchng. You have to be careful with “free” clip art or stock photography sites because the vast majority of them forbid the use of their images in commercial projects. stock.xchng is an exception, and the quality of images will surprise you. A very few of them have commercial restrictions, and some simply ask that you contact the contributor. Non-buyer beware. I can tell you that over 90% of the images I used were unencumbered, and the remaining onces required only notification, not permission.