Assuming you're using a PC, the freeware video editor Avidemux allows you to save high quality frames as either JPEG or Bitmap
Edited to add: It looks like you're using a Mac; they do make a version of Avidemux for the Mac but apparently it doesn't install as easily as the PC version. Still, it's free!
The older versions (Avidemux 2.5) also allowed you to mark a section of video and save it as a series of JPEG stills, but they discontimued that in the later versions. However I think you can still find version 2.5 on the Net. I have both versions installed.
Avidemux is dead easy to use.
You just open a video file with the standard "File-Open" menu or, just drag and drop the file across with the mouse.
Your keyboard left and right arrows step through the individual frames; the up and down keys jump forward and back to the nearest I-frame (0.5 to 1 second depending on the video format). The "[" and "]" keys mark the edit start and finish points respectively, so for example, if you choose "save video", whatever frames are between the [ and ] markers is what is saved.
To save a single frame: "File - Save as Image". Only one minor quirk; you have to actually add ".jpg" manually to the file name, it doesn't automatically add it, even though it says: "save as JPEG"!
Avidemux allows simple lossless editing if you always do your cuts at the I-Frame points. (Lossless in this case means the digital data stream is cut and spliced without decoding it. This is particularly useful for cutting the ads out of digital TV recordings, since there is no quality loss and rendering is extremely fast).
I've used it with 4K MPEG4 (so-called "Quad HD") from my Samsung Phone with no trouble. Avidemux is basically a windows front end for FFMPEG so it is a very versatile program.)
Here's a screen shot; that's me on the right holding the plate :-) (From the Masterfoods 2006 "Squeezy Sauces" ad campaign).