First, you have to get your image into digital format. Many of the PCCO classrooms and labs have scanners students can use. See below for advice.
Capture still images from a video with with video capture software.
Digital cameras allow you to shoot pictures like a regular camera, except the images are stored on disk instead of film. See the help file or documentation for the scanning software for help. Save the image in the appropriate format. Only 216 colors are common to both Macintosh and Windows. The image processing application may have an automatic setting for Web safe colors.