We use gitbook for collaborative authoring of lesson plans, and StoryBoardThat to create the comics.
After creating a comic strip in StoryBoardThat, download it both as a high-res image, and as a powerpoint presentation. Save the powerpoint presentation in Google Drive and open it as a Google Slides document. Make any edits you want, and then publish it to the web (example). Get the link and put it into the gitbook document.