The quality is limited by the resolution and color depth of the video display you are using when you do the capture, and that may be lower than the quality of the original. Screen captures are useful for illustrating computer processes - showing menus and buttons. I do it frequently in preparing lessons, but that's a bit specialized. The most convenient way to do this is with a graphics program such as PaintShop that lets you select the area of screen you want. You can then fiddle with it if you like, or save it directly in a Web format.
Most scanners look like small photocopiers attached to computers.
We can't go into detail of how to use a scanner in this module, but it's not hard. From the menu or tool bar, select Scan or Image Capture. this activates the scanner. Scanner software will give you a number of options, depending on the scanner. The results of the scanning will be appear in your graphics program, and you can either make adjustments or save it immediately in a Web format. Another source for Web graphics is digital cameras.