Many graphic designers are blessed with having large, high resolution monitors on which to design graphics (~20 inches, 1024x768 resolution or greater). A common mistake is to forget that most users will be browsing at 800x600 or even 600x480 resolution. Design for your users!
When scanning in images, higher resolution scans do not translate into higher quality output. Since pixel density is fixed, it only translates into a larger image:
|72 dpi||144 dpi|