Scanning
Contact the publisher and ask what resolution is suggested. If you cannot find this information, a safe amount is 400 dpi. As most printing uses halftone cells with at most a 175 line screen, any data beyond twice the line screen will not be rendered on the page.
Inexpensive scanners are often used for printing, but are not advised for professional publications. This mistake is often made because of resolution specifications. It's important to separate optical from interpolated resolution. Also, many economical scanners have different horiztonal and vertical resolutions—only the lowest should be used.
Resolution is just the beginning of scanning quality — more importantly is how accurately the image is recorded. Important issues include dynamic range and color fidellity.
If the image is going to be editted, it's best to double the resolution, again, to avoid signal loss from digital transformations.
Web & Presentation
Unlike print, the web doesn't have a set resolution. Depending on a viewer's screen resolution, the size may range from 72-100 dpi. Usually, there the size of the image is not controlled, and varies with the resolution.
By convention, 72 dpi is used, although most screens are set to a higher resolution. This is because the unit of DTP measure, the point, is equal to approximately 1/72 of an inch. (There is no halftoning, so images should be at the same resolution as the rendering.)
Like with print, if an image is to be editted, it's best to double the resolution to avoid signal loss.
