Gimp
https://www.gimp.org/tutorials/
File formats : https://www.gimp.org/tutorials/ImageFormats/
Raster vs Vector https://www.artworkabode.com/article/inkscape-vs-gimp https://docs.gimp.org/2.10/en/gimp-using-paths-and-svg.html
Gimp Components https://docs.gimp.org/2.10/en/gimp-colors-components-menu.html
https://www.gimp.org/tutorials/Basic_Color_Curves/
Transparency https://itsfoss.com/remove-image-background-gimp/ https://www.gimp.org/tutorials/Layer_Masks/
Antialiasing https://docs.gimp.org/2.8/en/plug-in-antialias.html https://www.gimp.org/tutorials/Anti-Aliased_Threshold/
Fonts and Text https://www.gimp.org/unix/fonts.html https://docs.gimp.org/2.6/en/gimp-tool-text.html
https://docs.gimp.org/2.6/en/gimp-concepts-text.html
Layers
https://docs.gimp.org/en/gimp-image-combining.html https://www.gimp.org/tutorials/Layer_Masks/ https://docs.gimp.org/en/gimp-layer-groups.html
Selections
https://docs.gimp.org/en/gimp-tools-selection.html https://docs.gimp.org/en/gimp-using-selections.html
Cropping
https://www.gimp.org/tutorials/GIMP_Quickies/ https://docs.gimp.org/en/gimp-tutorial-quickie-crop.html
Gimp ( Continued )
Blurs and shadows
https://docs.gimp.org/2.10/en/gimp-filter-drop-shadow.html
https://docs.gimp.org/2.10/en/script-fu-fuzzy-border.html
https://www.wikihow.com/Use-Drop-Shadow-in-GIMP
Objects and Paths
https://docs.gimp.org/2.10/en/gimp-using-paths.html
https://www.gimp.org/tutorials/Drawing_Shapes/
https://www.gimp.org/tutorials/Bezier_Selections/
Alignment
https://docs.gimp.org/en/gimp-tool-align.html
https://docs.gimp.org/2.10/en/plug-in-align-layers.html
Screen shots
https://www.gimp.org/screenshots/