The VRC’s images, accessed through the ImageBase, are high-quality JPGs that measure 1280-1600 pixels on the longest side. There are many variables that go in to presenting these and other images well, including the original source's image quality, your computing environment (graphics card, calibration, etc.), the quality of the projector, and the ambient light in classroom.
A quick guide to working with digital images:
A good rule of thumb is to use images between 1200-1600 pixels on the long side. If you would like to zoom you will need an image that is at least 1600 pixels on the long side. Bigger is generally better.
Note that while you can resize large images down to a smaller size, the opposite is not true. You cannot go from small to big without the image becoming pixilated.
Do not use tiff (TIF) files, as they are too large and may freeze your presentation.
If you use Google Images as your source, sort by "large images" to get good presentation size images. The dimensions will be listed under the image on the search result page. You can choose to search "extra large images" but will likely have to size them down for use in your presentation software.
Creating and editing:
The Art & Architecture Library has flatbed scanners available if you would like to scan your own images. A good setting would be 1200 pixels on the long side, scanned at 150 DPI.
If you have an unidentified image you’d like to find more information for, try a google image search. Go to Google images (http://images.google.com/) and drag and drop an image into the search box. Please note that this feature only works with Firefox.
This page provides a good discussion of DPI.