I've tried several methods to make a scanned signature look better in Paint.NET, and while I've been able to change the signature's color using the Brightness/Contrast method mentioned previously, I still wasn't satisfied with the image. After some experimentation, I found that "thickening" the signature lines is very effective and can be done with the following steps:
- Use the "magic wand" (or your preferred selection method) to select the outline
- Click "Layers"->"Duplicate Layer"
- Your newly created layer will be selected, so click "Layers"->"Rotate / Zoom"
- Increase the zoom level from 1.0 upwards until the outline is at the desired thickness. Somewhere between 1.02 and 1.05 usually works for me.
- After clicking "OK", click "Layers"->"Merge Layer Down"
- For added thickness/sharpness, use the "Effects"->"Selection"->"Outline Selection" tool. Note that this took is part of a plug-in pack.