I'll definitely have a look at that, although I was able to make the necessary conversion with Piccolo. A registration was required for JPG, but not for other formats so I used Imagecopy to convert the image to TGA or TIF or something before using Piccolo to convert to a 16-Bit IMG. Imagecopy confirms the IMG now has a 65535 color palette, and the file size was much smaller.
So, with this I finally could test several of the programs to see if they would accept this as a desktop background / wallpaper. Unfortunately, it seems that Deskpic and Jinnee only accept up to 256 colors, and Picdesk does not allocate enough memory for a 640x480 true color image. I can get a 320x480 image to load (24-Bit I believe) but not the whole screen.
I suppose these programs didn't anticipate that people would have a 640x480 true color desktop :/
But now, an answer:
Didier Mequignon's Picdesk does support True Color (X)IMG files. Still, by default it allocates 500000 bytes, which is not enough for a 640x480 24-Bit (X)IMG. The answer is a small edit to the Picdesk.prg using a sector editor. I used good old Disk Doctor:
> ...since some years it's possible under a Falcon to change the size of
> this buffer (500000 bytes). This size is stored at the beginning of the
> PRG at the byte offset 32, there are normally 0007A120 (500000).
> if you have a tool for patch the PRG, it's easy. Else if the size of the
> picture is too big or the sreen format is different you can't see the file
> inside the file selector of the CPX.
To double the size to 1000000 bytes, I replaced 0007A120 with 000F4240, and success! A beautiful desktop
Thanks again for all of your suggestions!