Inserted photos have the text '{C}' added before and after their captions. It happens every time a page or comment is edited, so after several edits there will be a long string of {C}{C}{C}{C}{C}{C} added.
The current workaround is to turn off the photo's caption (right-click the photo, choose 'Edit atom properties', untick 'Add a caption', click OK.) You can still add text under the photo manually if needed.
I've traced the problem to CKEditor, the software that provides the edit box when we're editing text. I've reported the bug (https://github.com/ckeditor/ckeditor4/issues/4777) and their team confirm it is a problem with their software. Once a fix becomes available, I'll update Gwulo's copy.
CKeditor have fixed the bug
CKeditor have fixed the bug and the Gwulo website is now running the updated version of that software.
If you see the {C} shown in any of the posts or comments you edited while the bug was active, please consider editing them to delete the {C} from the image captions.