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.
