Your uncle's friend's cat knows this dude who sells fur hats and wants you to put together a web site. Except, all you have to work with is photos from an old battered-up catalogue. Here are a few tips you can follow in PhotoShop to make your scans come out crystal clear on the web. Apply these tips by themselves or as a group as needed.
The Scan: Using any off the shelf scanner, scan your images into PhotoShop. If possible scan them in at about 200% of their original size. Of course your not going to be using these huge images in your site, but it is generally easier to remove dust and scratches from larger images than smaller ones and will give you more flexibility with any digital altering you may want to do.
Stamp it, don't scratch it: One often-neglected tool in PhotoShop is the Rubber Stamp Tool. This tool is very easy to use and can help you remove nasty scratches and large foreign objects from your images. Select the Rubber Stamp Tool and while holding down the ALT [option on mac] key click on your image where you want the Stamp to take the impression from. Now release the key and use the tool like a paintbrush over any area you want to 'stamp it out'. Play around with this tool first until you become comfortable with how it works. Then go to town on your images.
Despeckle and Sharpen: Notice how sometimes after you scan your image it looks a little rough or grainy? This can be caused by the texture of paper you were scanning and can sometimes be very noticeable. One fast way of cleaning this up is by using the Despeckle and Sharpen filters. FILTERS > NOISE > DESPECKLE, will clean your image but may also leave it looking a little out of focus. You can counter the focus with FILTER > SHARPEN > SHARPEN. Note: If you plan on doing more work to the image it may be a good idea to leave the sharpening until you're done.
Brightness and Contrast: Ever wonder why some images look a little flat or lack life after they are scanned. A big reason for this is not having enough contrast between shadows and highlights. An easy way to give your online images a little kick is to pump up contrast slightly to darken your shadows and bring focal points into more light. The brightness/contrast control can be found under IMAGE > ADJUST > BRIGHTNESS / CONTRAST.
Resize and Sharpen: You've adjusted the contrast, cleaned the scratches now it's time to give your image one last tweak before exporting. A nice fast way to bring that image into focus and give it an ultra clean look is to shrink the size down to where you need it and give it a quick sharpen. FILTER > SHARPEN > SHARPEN.
Now you can save your image and dump it onto your page. Remember to play with your compression settings to find the perfect amount for your image, any more loss to quality at this point will result from poor saving practices or poor file format choices. For some information on file formats and optimization see the evolt article Optimizing Art for the Internet.