A short while before the summer 2017 break, Nick Jeans, Senior Consultant in Learning Technology at Sero Consulting, enquired in the Association for Learning Technology discussion list whether anyone had any recommendations for free comic-making software. I've collated the responses here. (Nobody in the discussion list has objected to my plan to do so.)
You'll need to check for yourself whether all of them are still working or free, but even if one or two have gone the way of all flesh, there is still a lot to choose from!
James Clay has also reviewed various free and non-free comic apps at http://elearningstuff.net/tag/comic/ – definitely worth a rummage!
The following were collated by Oriel Kelly:
Synfig Studio (An animation software but can easily export as still comic frames)