So I'll admit it, for most of my writing career I have created my book covers using either PowerPoint or Google Slides. Ironically it's really easy to make a simple cover in there, but sometimes you need more than that.

So a few programs I recommend are:

Krita

Gimp

Darktable

Photoshop Express

Cyberlink Photodirector (If  you don't have Photoshop)

Google Sheets

Inkscape

So what are these programs? Krita is a open-source drawing program, GIMP is in it's name a GNU Image Manipulation Program - so open source, Dark Table is an open-source highly powerful image editor, Photoshop Express is photoshop lite, Cyberlink Photodirector is photodirector for PC, and Inkscape is a .svg program editor.

Now, personally I use three programs: Photoshop, Lightroom and GIMP. These are my main editors and when I am ready to export, I bring all of the photo elements into either Sheets, LibreOffice Impress, or Powerpoint to do the final export.