Darktable has a bit of a learning curve, but in its latest update to 3.6, it’s easier to use that it ever was.
YOGA Image Optimizer is a free command line tool to convert and optimize images from various format to JPEG, PNG and WEBP.
Inkscape 1.1 released with huge set of improvements. It's a free vector graphics editor used to create vector images in SVG format.
Drawing is a free basic image editor for Linux. It is similar to Microsoft Paint, but aiming at the GNOME desktop.