Ok if you want to add transparent background to your picture or png files in linux these are the steps you need to follow
1) First Open the image with GIMP
2) Go to layer-> Transparency -> Add Alpha Channel
3) Now click on the fuzzy selector tool and click on the area of the image which you want to make transparent.
4)Now go to Edit –> clear as shown in figure
5) Now you can see the final image after making the background transparent.
Cheers,
Have fun with gimp