I don’t know of any web-based editors but there sure are.
Gimp is an image manipulation software free available for GNU/Linux, macOS, Windows
Krita is a free open-source painting program also available for Windows, Mac, and Linux.
Then you have Adobe Photoshop but I think that one is paid and not available for Linux.
If you want to draw things yourself like from scratch, you most likely than not would need to get a drawing pad.
(If you want me to draw something for you, I could maybe help you with that.)
I’ve seen people here in the forum mention some “free to use stock image pages” you can also use to create your own mashups.
For chapter headers here is an excellent guide one of the forum members has made. It also uses Gimp so it’s completely free.