A good rule of thumb is to use images between 800-1200 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 pixillated.
Avoid using tiffs (.tif), as they are usually too large and may freeze your presentation.
If you decide to use Google Images, 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 you will likely have to size them down for use in your presentation software. Also, note that the authority with which images are identified and described varies considerably, since Google Images locates files from the Web at large. Databases that are created by scholars and regulated by cataloging rules are much more trustworthy.
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.
You can use free photo editing programs online to downsize large images.
Try these: http://www.picnik.com ; https://www.photoshop.com/express/landing.html.