find . -name "*xcf" | xargs -l -i basename "{}" ".xcf" | xargs -l -i convert "{}.xcf" "{}.jpg"
https://forums.linuxmint.com/viewtopic.php?p=1089649#p1089649
outra forma - https://gist.github.com/danstuken/3625841
1. Open your image in the gimp.
2. Right click the image and go to LAYERS then ADD ALPHA CHANNEL. You won't notice anything happening, but don't be concerned. It basically adds a transparent layer at the bottom of your image so when we erase the colors.....it's shows the transparent layer. Which of course would show whatever was under it on the screen.
3. Right click on the image again and go to SELECT and then down to BY COLOR. A window that is all black opens up. Don't change any of the settings....just use the defaults for now.