To scan an image, first open photoshop.
In photoshop, go to File>Import.
Select CanoScan 4200F
A window will pop-up that looks like this:
On the left, select Advanced Mode.
The tool bar will change to this:
Here, you can change the color mode (Greyscale or Color), and more importantly, the Output Resolution.
Output Resolution is where you determine the size of your image in dpi, or dots per inch. A good median setting is around 150dpi, which gives you great resolution without making the image gigantic. 300, which is the default, is a little excessive. As a frame of reference, video screens max out at 72dpi. It's always good to go higher, though, giving you some room to play around with the image. You can always shrink it down when your done.
Preview allows you to see how your image is situated on the scanner. You can then drag the constraining box around the image, and hit "Scan".
Once the image is scanned, it will open within photoshop. Next, simply go to File>Save. You should be OK in just saving it as a simple .jpeg.