For an image for web, or only for looking on a screen, the resolution is 72 dpi. When you scan an image, the higher the resolution, the more details you'll have. The filters you'll apply will have better results. It's common use to scan at, say, 600 dpi, work on the image, then go down to 300 dpi for printing. That resolution has been choosen because it's roughly 12 line per mm, which is the eye resolution. But there are cases where the output media is resolution fixed, as a screen, where it is 72 dpi. When you use an photo from a library, say a 30*45 cm @ 150 dpi, if you need the size of the photo to be smaller, don't resample and so the resulting resolution will be higher. You are keeping the details which are there. Last,if you have an image at 72 dpi, on push the resolution to, say, 150 dpi, the quality will stay the same. The file will only be bigger. You can't invent details...