This is very easy and uses only software included with Mac OS X.
1. At required place in movie select Edit -> Copy.
2. Open Preview and select File -> New from Clipboard
3. The photo should now open in Preview where you can save it as JPG, PNG, PDF or TIFF.
You can use Preview to edit the photo before saving if you wish, or just load it into your photo editor of choice.
Of course as long as the photo editor has similar feature for creating photo from clipboard
you can skip Preview and create in that editor. All my photos are first
edited in Lightroom CC and I import the photo saved from Preview into