The Gimp

How to Sharpen Photos

First decompose the image into HSV: Colors/Compose.../Decompose. Select "HSV" from the list and "Decompose to Layers". This will make a new image (close the original).

Select the "Value" layer for editing (so that colors aren't affected). The "Unsharped Mask" filter is under Filters/Enhance.

Radius is how many pixels to select, amount is the strength of the filter, and threshold allows parts of the image to be ignored.

After applying the filter go back to Colors/Compose.../Compose and select HSV again to regenerate the color image.

How to Use Transparency

Layer > Transparency > Add Alpha Channel