Usually, when you have the file open in something like Microsoft Photo Editor you can do a File...Save As. Then choose a format that has compression (jpeg would be a good choice). If you choose jpeg, you'll either have or can select (in Microsoft Photo Editor there's a More... button to display advanced settings) a setting that saves space over quality.
For example, here's a picture of my '85 R80. In the original high quality JPEG format it's about half a meg in size. Setting the size/quality setting down yielded a file of about 90K and still made a usable picture.