There are a very wide variety of solutions for resizing and sharpening images for use on the web. As preparing images for publication on the web generally involves a reduction in size, you have the benefit of starting with more information than you need. This is always the better position to be in. When reducing an image, use a Bicubic filtering method, rather than a Bilinear, as you'll get better results. (In photoshop, Bicubic or Bicubic Sharper is best.) Sharpening is usually a key factor in producing a quality reduction. Exactly what you are sharpening will sometimes determine which approach you take to sharpening, as not all content is equal. Different approaches work better for different kinds of content.

Sharpening for print is generally more difficult, as if often requires increasing your image size, meaning you start out with less data than is necessary to produce a quality print. It is possible to scale an image up and still maintain high quality, however. When scaling and image up, you can either do it in a single step, or do it in multiple increments. Scaling up incrementally will usually produce better results, as you are iteratively adding new information, which provides each successive resize with more information than if you did it all in one step. Between each resizing step, or perhaps every other step, you might want to sharpen your image to "generate" even more useful data that may be used when resizing. Again, sharpening is a key factor when scaling up, however the process will generally be more complex if you wish to maintain the maximum amount of quality possible. Regardless of what you do, an enlarged image will never contain the same maximum degree of quality as the original.

Image upscaling is an area that is constantly enduring rigorous research, and improvements are made on a fairly regular basis. Many third-party tools exist that employ very advanced algorithms to scale your images (up or down) while maintaining the maximum amount of quality that can be. They often cost a pretty penny, however if you have the cash, they will save to a ton of time (and, depending on the volume of work you do, possibly a lot of money too.)

There are a wide variety of techniques that may be used to resize and sharpen images. Here are links to a few that I have found helpful in the past:

• Processing and Saving Photos for the Web
  • Good general ideas, primarily geared towards nature photography
• How To Sharpen An Image In Photoshop — Advanced Photoshop Sharpening Techniques
  • An advanced technique that shows how to custom-sharpen an image in only the areas that need it
• Cambridge In Color: Guide to Image Sharpening
• Cambridge In Color: Image Resizing for Web and Email
• Cambridge In Color: Sharpening with Local Contrast Enhancement