I've done it in iMovie but I think you can easily do it in any video editing app. First, trim the video down as small as you can isolating the frames that contain the frame that you want. I was able to trim a video down to 0.3 seconds in iMovie. Then save the trimmed video to Camera Roll. Next, open the trimmed video and display it "full frame" on your screen. Tap the exact spot of the video in the timeline that you want to capture so that it is displayed full frame on your screen and then do a screen capture (by pressing the Home button and Power button at the same time for just a quick moment) and the frame will be saved to your Camera Roll.
Originally Posted by Elaine5006
I'm unclear about your statement about "none that support that kind of video". That might be an issue.
Let me know if you have questions. Here is an example of a captured frame using the process above: