I read through all the comments and responses. @harfang@slrpnk.net you need to make slides quick and easy using drag and drop and font layouts. That's why you use Canva from what I can tell. So getting hardcore image manipulation is not necessary, nor collaborative style programs.
You can, with minimal effort, use Libreoffice Impress. Its PowerPoint without the Microslop. What this requires is you find a good source for properly licensed fonts and images, places like Pixabay and such.
You can use GIMP if you want to, but it reads like you just need to learn to use the tools in Impress. Again, GIMP is basically a Photoshop replacement. I'm not sure you need that.
Here is my work flow I used to make pretty slides and lecture decks that rely on prompts and imagery for students, not death by text:
-Outline in Libreoffice Writer. -Search for images on Pixabay, Unsplash, or Wikimedia Commons. -Search for, or use incorporated, fonts. Two to three font faces. -Build a draft presentstion in Libreoffice Impress -Set it down, walk away for a bit. -Return and edit as needed.