Steps for cropping or cutting parts of image using gimp in Linux
1) Open the image in gimp
2) Select the cropping button as shown in figure 
or go to tools–>transform tools –> crop
3) Select the area of the image you want to crop
4) Click Inside the selected area and you will see the crop image and save it.
Use GIMP to scale images in linux. Gimp is user friendly and very easy to use.
Scaling or resizing images in gimp is an easy task.
1) Open the image in gimp
2) Click on the scale button and click on image 
or go to Image –> Scale as shown in figure
You will see the options as below. Now change the size you want to scale the original image and click on save button. Now save the image.
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