We use images all the time in our classrooms and technology makes it increasingly easy to find pictures / flashcards etc. and also to manipulate them and create customized material. You and your students may be familiar with http://www.flickr.com/ where you can store your photo sets online. I have just found, however, http://bighugelabs.com/ which offers a series of tools for manipulating photos, creating a jigsaw for example or a mosaic or poster from files you can upload or import directly from facebook or flickr. Great fun and very useful for creating stimulating images for use in class: try getting students to do their own and bring them in for discussion.
If you want to use pre-selected photos there is nowhere better to start than with http://www.flickr.com/photos/eltpics/sets/ the great ELT picture project. Have a look at the photos already available and well done to those involved! As a continuation follow the great new blog with ideas on how to creatively use photos in your classes http://takeaphotoand.wordpress.com/
Great work by all involved!