Changing icons for mpg files


I am in the habit of changing the icons for my .MOV files by selecting a frame while playing the movie and then hitting apple-c, then get info for the file, selecting the icon, and hitting apple-v.

The snag is that this doesn't work for my .MPG files. Is there a reason? This a bug or a "feature"?
'The snag is that this doesn't work for my .MPG files.', it never did.
'Is there a reason?', yes - Apple does not allow such action ... directly.
'This a bug or a "feature"?', no - not a bug; nor, a feature.

01. Locate the frame you want, as an icon, in the '.mpg's QuickTime window.
02. Click on the 'Desktop' to bring 'Finder' forward.
03. Do a snapshot ('Command <shift> 4'), and then select the frame portion of the QuickTime window - with the crosshair cursor. On the 'Desktop' will be a file titled 'Picture 1.png' (the extension may / may not be displayed).
04. Use 'Preview' or another Graphic application (such as 'GraphicConverter', GC) to view 'Picture1.png', do a manual selection ('Preview') or 'Select All' ('Command A', with GC) and then copy ('Command C').
05. Locate and select the '.mpg' file whose icon you want to change - do a 'Get Info' ('Command I'), press the <tab> key, do a paste ('Command V'),
I've tried that before to do the screenshot route and paste it in. For some reason, even though the copy command is working (I can create a new preview document from clipboard), the paste command is greyed out in the get info window for all .MPG files. Really frustrating.