Having no artistic ability, and never having used graphics software professionally, I don't know much about GIMP but it is well-esteemed. YouTube will have many tutorials.
Plus, I never really see the need for editing image files other than a simple bit of cut and paste. Why do people want to edit family history image source documents? You wouldn't edit a census form or a BMD record.
Zaph
Yes Zaph one would edit an image of census form, just as you edit the scanned image of old family photos.
For say an image of an original census form, many can be very difficult to read and by using a Photo Editing App the image can be enhanced to make the text far more legible.
Being the sad person I am there are digital images on my PC of all the census records for each direct ancestor of mine, and each image has been enhanced to be as clear and easy to read as possible.
Same process with Newspaper articles and any other downloaded or screenshot of a document that was the subject of a poor quality scan or photographic image.
What I hope will be of use to future generations is the PDF Booklets that I have been creating for each Grandparent line and for individuals with an interesting take to tell and the applicable images as described above are used in the creation of the Booklet.
So yes Photo Editing Apps and a Publishing App can be very useful tools and well worth the time and effort in learning how to use them.
That said with an iPhone/iPad the inbuilt Photo Editing App is quick and easy to use and very good for enhancing the legibility of text.