Canva's Comic tool, with loads of great templates is fantastic. Start from a layout and you can drag in your graphics.
A wonderful collection of older strips, many of which are public domain.
For fun, you might, control(mac) or alt(click) to save them to your computer and edit them with Pixlr to add your own words.
Lots of free digital comics in PDF for you to buy for £0.00.
The Internet Archive has a section of