Command line image editing
Two command line tools to remember are
two similar, stand-offish rivals.
Imagemagick is the “original” whatever that means in the confusing phylogentical world of software.
Graphicsmagick has, AFAICT,
a better API.
They are often farily interchangeable from a user perspective.
[Fred Weinhaus’s imagemagick scripts) show you how to do a great many useful things, e.g. Histogram matching. (⚠️ Beware the odd licensing conditions.)
convert favicon.png -bordercolor white -border 0 \ \( -clone 0 -resize 16x16 \) \ \( -clone 0 -resize 32x32 \) \ \( -clone 0 -resize 48x48 \) \ \( -clone 0 -resize 64x64 \) \ -delete 0 -alpha off -colors 256 favicon.ico
convert favicon.png -define icon:auto-resize=64,48,32,16 favicon.ico
GUIs? you want, e.g. the adobe suite, which I will never touch because of the economics of comparative advantage…
Or a weird alternative one.
Gimp is a flagship open source image editor. It has powerful plugins. Its user experience is a clusterfuck; there are such startling and unneccessary pain points as the fact that plugin registry (which is IMO the only reason you would want to dive into this dorknado) is broken in an undocumented way that probably has to do with it being am undermaintained security nightmare.