See that “Export Frame To Pictures” option? Yeah, that’s all you’ve gotta do. And as you can see, there are some other handy choices there. “Trim” is obviously useful (as is “Reset Trim” if you’ve munged a video up when you tried to edit it). “Set Poster Frame” will change what thumbnail Photos shows on the selected video when you’re scrolling through your library. And the final option under that menu will let you swap back and forth between scanning and stepping controls, so you can either step through a video frame by frame or scan forward and back in the more traditional manner.
Oh, and one more thing—did you note that the “Export Frame To Pictures” menu item says, you know, “Export Frame To PICTURES”? Don’t be like me and expect it to show up in your Photos library. The TIFF for the frame you exported will be within /Users/[Your Username]/Pictures.
I definitely did NOT spend 15 minutes scanning my library looking for that silly image before I realized my mistake. Nope. No sir.