I'm not on DA anymore, but I found some (very old) requests for my images.
Yes, you can use them as preview images, for school work, backgrounds, whatever, as long as you're not making money off them. You can link me or not, notify me or not... up to you. Basically, you're free to do whatever you want as long as it's non-commercial. If you think you can use them for something commercial, send me a note and... well, hope I check my DA in a timely manner.