What does the 'J' icon on a photo in the Photos app mean?
Source:


  When you shoot RAW + JPEG the Photos app stacks the two images together. The J means you have a stack with a JPEG file and a RAW file and the JPEG file is used as original. To change to the RAW file go into edit mode (enter) and choose Image - > Use Raw as original. The letter will now change to a R.