The best way to improve a poor quality image, albeit a bit work intensive, is to resample/resize* it. For example, if you have a 512x512 image, resample* to 1024x1024. Now the same image has 4 times as many pixels.
Then, you can apply, either image wide or with a tool, a sharpen mask/effect. Tool is better, because you can control the depth of the sharpen, especially if you have pressure control, and you can be more selective about which parts of the image gets sharpened and which don't. You can then use whatever other editing tools (soften, blur, etc) to clean up the image as you see fit.
*Most graphics sw that has resize/resample will include an "Interpolation" setting in the effect, such as "linear" or "Bi-cubic" etc. Experiment with each of these to see which works best. If the sw warns you that the effect is "not undoable" then make several copies (imagea
.jpg, etc) before applying this.
Edited by NQx, 05 February 2007 - 05:23 AM.