Photoshop Making Low Res Look HighPhotoshop - Making Low Res Look High
Low Resolution on a Grand Scale: Making Low Res Look High
Understanding the Spec Sheet
Assessing the Situation
Improving the Existing File to Survive Magnification
Dealing with Specific Image Problems
The first reaction is always scanning or reshooting the image at a larger size. If this option exists and can be done, it's idealbut this isn't always the case. Some images may have been purchased or shot in some exotic location where a reshoot is financially unfeasible. And sometimes, even if a reshoot or a new scan is an option, the retouching may still need to be repeated. If the retouching was very complex, it can be rather frustrating to redo the work or quite difficult to replicate what you have done the first time around, especially if you have a memory like mine: it's very good, but short.