Hi, welcome to the community! I’ve moved your post from Software Boutique and add-ons to a new topic so the wider community can assist you.
Installing GIMP is actually as simple as opening the Software Boutique, you can find this in the System → Administration menu. The icon looks like a coat hanger.
Once inside, choose the Graphics category (its icon is a paint brush) and then scroll down until you see GIMP or GNU Image Maniuplation Program. Clicking Install will add this application to your system along with a collection of additional plug-ins.
Once installed, it will appear in your Applications menu from the upper-left of the screen in the Graphics section.
Let us know if you need any help!