do mean the "fog"?
You do this:
1. open gimp and create new picture
2. right click on the layer, called background, and click add alpha channel
3. press ctrl-a to select everything then press delete, so you should see a checkered background
4. select the paint brush tool
5. in the options set the opacity (clearness) to something lower than 50
6. when you stroke it on the picture, it should be partially see-through
7. play around with those settings for different results
long post ftw