GIMP
GIMP (GNU Image Manipulation Program) is a Free & Open Source Image Editor. GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. It is free software, you can change its source code and distribute your changes.
Whether you are a graphic designer, photographer, illustrator, or scientist, GIMP provides you with sophisticated tools to get your job done. You can further enhance your productivity with GIMP thanks to many customization options and 3rd party plugins.
Being a newbie with these kind of programs I did not know where to start learning how to use it. In the following sections I mention some links that I used to get an idea about its workings.
YouTube lessons in Dutch
Slow paced lessons (just what I need) by Mariëlla Wassing on YouTube. Mariëlla also created a course on the Soofos platform. Another (?) version of the course can be found here.
If you prefer text instead of video you can have a look at this website.
Have a look at the Dutch GIMP Forum when you have any remaining questions after viewing the previous resources.
YouTube lessons in English
For a YouTube introduction about GIMP in English see the series by Jan who has more examples in playlists.
GIMP book
Grokking the GIMP by Carey Bunks