Personally, I’d say finding the right resources to use is the trickiest problem. Some commercial stock photo services have elaborate ways to ease out the process, but it remains difficult. Part of the solution could be through the Semantic Web.

In fact, I work for Vitrine technologie-éducation, a non-profit also connected to Quebec’s college system. Ceres, our OER catalogue, is itself part of the Semantic Web. It indexes thousands of these CCDMD resources (along with many others), using a local version of the Learning Object Metadata (LOM) standard.
http://ceres.vteducation.org/?lang=en

In the next few months, we’ll be working with teachers on an exploration of OERs. Chances are that looking for relevant visuals for use in educational material will be among key issues we’ll tackle.