Before you start searching for images, it helps to categorize the way you want to use a digital image. For research? web site? printed brochure? poster? holiday card? Once you have a firm concept of your use, you'll have a better idea of what resolution you'll need, whether you can use images the university has licensed, or whether you'll need to negotiate your own license or other use agreement.
Broadly speaking, there are two kinds of digital image collections: Free and Licensed.
Each digital image collection includes one or more types of images, such as: