I have long time used GIMP as my main image manipulation tool when making images for ePanorama.net. It can do all the normal image manipulation needed well. GNU Image Manipulation Program (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. GIMP provides the tools needed for high quality image manipulation and drawing bitmap images. GIMP also is used for producing icons, graphical design elements, and art for user interface components and mockups. GIMP is celebrating that it is already 25 years old. There have passed there are are several years since I have mentioned GIMP in this blog.
There are fee GIMP tutorials available at https://www.gimp.org/tutorials/