Hi.
You can check Ajay K's answer on this link on How to capture
a still image from a video mp4.
Regarding your second question, are you referring to the Movie Moments app? the moment.jpg might be the name of your file that's in .jpg format.
Also, A JPG file is an image file that uses a compressed image format standardized by the Joint Photographic Experts Group (JPEG). It is commonly used for storing digital photos and used by most digital cameras to save images. JPG files support up
to 24-bit color.
Regards.