I have used Pics4Learning--it comes from the Tech4Learning (where I was nominated as a T4L Innovative Educator) company. I have used this with their Pixie creative arts software, along with other projects I have worked on with students.
Photl.com has 160,000 copyright-free images available.
The University of Victoria Teaching Clipart Gallery has three thousand images specifically designed for language-teaching.
The Royalty Free Clip Art Collection For Foreign/Second Language Instruction from Purdue University is another place to find images useful for teaching English Language Learners.