J6I is a format used in Ricoh digital cameras, and was once in common use, though not any more. It is one of the many formats for which any attempt at a search for details results mostly in scads of the cookie-cutter sites that sprout up like kudzu to "document" every file format, but are exceedingly short in actually useful information. By putting this page here, we've probably added one more not-so-much-information page to that bunch, but at least it's a wiki so that if you do happen to have a bit more to say, you can edit it yourself to add it and help future information-seekers.
The format is apparently related to JPEG, but given some proprietary twists.
J6I files are created by Ricoh RDC-2 and Ricoh RDC-2E. Maybe other models too. Format was in use around 1998. It seems this is just a JPEG with a custom header added before it. The header contains date. The image is saved in weird proportions 1:2. Which means the height needs to be doubled in order to display properly. It's possible that Arcsoft's Photostudio 2.0 can convert JPEG images into J6I. Ricoh RDC-2E captures images in resolution 768 x 576.