Re your second method, did you copy and paste it correctly? You need to open the Get Info dialogue box and select the thumbnail icon at the top of the box. Then open your jpg photo and do Edit/Copy, and then Edit/Paste it into the Get Info icon. That will get you the jpg poster frame on the movie file on your desktop. However, it will not carry over to other apps, including the photos app. But it will transfer to an external drive if you move the desktop movie to it. In other words, it will work on your Mac Finder system but basically nowhere else. This is because you would have changed the file data of the file but not the meta data.
If you upload the movie to You Tube, you will have the option to change the poster frame within the You Tube system, but I don't know if it would carry over to other apps. Probably not.
iMovie sets the poster frame as that of the frame that appears in the middle of the movie. So you could change the poster frame by, say, adding a lengthy black background at the end of the movie to change the center point.
Otherwise, I am not aware of any way to change the actual meta data to create a poster frame that would carry over everywhere. I suppose that there are apps out there that could do that.
-- Rich
