The Fine Arts Library DLA site lets you limit your search only to images whose quality is suitable for PowerPoint.
Advanced image search on Google lets you look only for small or medium images.
An image of this size needs to be resized.
This University of Michigan primer explains the concepts of resolution, and what to aim for in PowerPoint presentations. It also explains the resolution differences between print, PowerPoint and web.
Some technical tips on using PhotoShop to automatically improve a set of images for display through a high-resolution projector. This may be more detailed than needed for this project.
Microsoft PowerPoint 2007 Tutorial explaining the new meanings of themes and templates.
Article on options for creating picture frames to put around your pictures in PowerPoint - using PowerPoint or PhotoShop, or downloading images.