Another idea might be to just copy the iso file into the USB stick and then boot it with GRUB.
Boot a live CD iso on your hard drive using GRUB2
That's what I do and I also install GRUB to the USB and add GRUB files and a grub.cfg and change the ownership of it so it's quick and easy to edit with pluma. A lot of the stuff in it can be just deleted out of the menu section if you're not using it to boot your main installed operating system(s) and replaced with you iso booting stanzas.
The USB can still be used for files that way and there's no need even to format it first. GRUB understands all file systems as far as I know. Most iso files won't take up much space in your USB and so it's possible to fit several more iso files in the same USB. When a newer version's iso file comes out you just delete the old one and edit the grub.cfg with the new one.
I realize this idea might not be the best for everybody, especially anyone with a fear of the command line, but there are some people who might find it simpler and easier in the long run.