actually the folder.jpg thing does not work anymore. You have to take a jpeg file, call it 0.jpeg (use the number 0 because it reads the first one in the folder). Then rename it to 0.metathumb. This should work.
In regards to the ‘folder’ jpeg - it will not work if you have metathumbs within that folder. What happens is that the player looks for the first metathumb within the folder and displays that. Yes, if you do have metathumbs in your folder you need to name a metathumb something like “0” to get it to display.
This has always been the case and has nothing to do with recent firmware updates.
I’ve had limited success with the “1.metathumb” approach. It may work for a while! but the SMP seems to eventually revert back to the alphabetically first “real” metathumb associated with a video. I am likely doing something wrong but can’t figure out what. I use a jpeg image and I name it “1.metathumb”, which seems to be the recommended approach. To be clear, the file is still a jpeg, correct? By this I mean the full file name is 1.metathumb.jpeg - I’m not actually converting the jpeg to a metathumb. This file name looks different than the metathumbs associated with videos, which don’t have any jpeg in the file extension. If there is a way to drop the jpeg extension then that is likely where I am going wrong - any suggestions appreciated.