I didn't know either that GIMP could open & modify pdf files. I am glad you mentioned that.
I have had great difficulty just changing the background color of an image in GIMP. If you have found a specific GIMP book to be so helpful that you bought the book, please post its name, author, and publisher. I keep googling how to do something like what I mentioned in GIMP. But it is never simple. Like for example, how do I crop an image ? I have done this a couple of times with GIMP, but cannot remember how to do it. I have to google how to do it whenever I need to, and then sort through which website will actually help me get the task done.
joany, thank you for mentioning the convert command. I have never used it. II don't know why I'd ever want to convert a pdf to jpeg, but I'm glad to know I can do it in BASH.
If anybody knows a simpler Debian program I can use to easily crop images or to easily change an image's background color, say so. Gwenview doesn't have those functions.