Scan image at 300dpi and save the file as in TIFF format (uncompressed). This should be your archival file and is usually what publishers request for publication. Never use tiffs in PowerPoint or in other presentation software as they are too large.
Using an image editor, resize the TIFF to 1024x768 and save as a JPG for on-screen presentations in 72 dpi. 1024x768 is the output resolution of our highest quality digital projectors. To create a detail, crop the TIFF, but make sure your crop is not much smaller than 1024x768 if you want it to fill the screen, then save as a JPG.
Computers in the Art Library and the labs at the Dodd have Adobe softwares installed, including Photoshop, and you can use them! Additionally, Preview on a Mac offers some light image editing tools--adjusting color, image resizing, cropping, rotating. Pixlr is a free online image editing tool. Picnik is an online image editing tool that works well if you store your images online already through Flickr, Picasa, etc.