First thing to look at is the image resolution or dimensions. If you scanned the image in, the chances are that the dimensions are in the thousands. Depending on the image and what it is for you want to aim for something under 640×480 for basic graphics, and something between 800×600 to 1024×768 for pictures.
Another setting to look at is your DPI. Images that are print ready run about 300 DPI or higher. This adds a great deal of information that is not needed for websites. For web usage set your DPI to 72.