GIMP is a free and open-source raster graphics editor used for:
image manipulation and image editing
free-form drawing
transcoding between different image file formats
...as well as more specialized tasks.
How to open Gimp at school
Click on the search icon (found on the bottom left of you computer screen)
Type in Gimp and it should already be installed on the computer
If you have your own device that is not a Chromebook you can get the free download here.
However if you have a Chromebook that can run the Linux (beta) see here for instructions to enable it - and then get GIMP for GNU/Linux here.
This is a written and video tutorial getting you started with some of the basics of GIMP.
Simply click the picture on the left.