Please check this out:
The preview display setting has influence on the size of the snapshot file (hence the quality?)
Obviously snapshots taken form the timeline....
I have used the PD sample clip Skateboard 02 for this test.
For JPG files is see:
1) The size of High preview and HD preview are the same
2) the size of the full HD is lower
3) the size of the UDH is more than 2x larger the size of 1)
for PNG files applies (my tests)
1) the size of High preview, HD and FHD are the same
2) the size of UHD is 3x larger than the others.
It is not possible to set set the preview quaility for clips in the media room.
However, the size of snapshot in jpg files taken from the (same) clip) in the media room and on the timeline are about the same, but still smaller than the once taken from the time line with UHD.
Given the way JPG and PNG files are build I am sure that the results might differ for clips of another content.
My conclusion would be for now, and only based on the size of the files: Take a snapshot from the time line with the preview setting to UHD.
Other conclusion could be, that the way PD takes snapshots differs and depends on settings. Would it not be a good idea that PD always takes the highest possible quality settings for any and all shapshots?
(I will share that suggestion with CL)
I am looking forward to see other results.