Works very quickly and with a very good quality. If you can't launch it: RTFM (https://gitlab.mister-muffin.de/josch/img2pdf). Simply open a terminal and use it like this: "img2pdf img1.png img2.jpg -o out.pdf". You can also use "img2pdf --help" if you need some help :)
At first I thought it would take too long to create a PDF from, let's say, 14 JPGs without a GUI. However, it takes surprisingly little time. I set Nemo and terminal next to each other, type "img2pdf" and then drag all the pictures, one by one, to the terminal. At the end I add "-o something.pdf", hit enter and in a moment PDF is created. Of course it would be nice to have a simple tool with GUI, but still img2pdf is very nice.