imo, it depends on:
- what quality do you need with these images?
- is your project commercial? (i.e. are you able to pay for images)
- how fast do you need to get graphic content?
If you need high quality and right now, you have to try one of the pay microstock websites - millions of ready-to-use images which are properly and clearly licenced.
If you do not want to spend a cent - use free sources, but be aware that they to not pay much attention to image sources and licensing.
Anyways, you may check the website in my sig - i have lots of resources of both types there for many content types. Hope, it helps..