It isn't hard to extract a still image from a video. There are good reasons why you want to do this.
One reason could be that you have a video of an event and you need to transition from one scene to another. A still images can be extracted and used as an transition point from shot to shot.
Another reason you want to do this is that some of the video web hosting services requires a sample image from the video to use as a poster or advertisement. If you don't provide a photo the service might electronically do that for it for you. You might not like the frame they have selected to represent your video.
Still images not only can be used to enhance your videos you might find that you have a great photo that you want to print. The quality of photo still image is going to be dependent on the quality of the video.
Copying an Image or Frame
One place to look for this feature is your video editing program. For example, in Serif MoviePlus5 I can extract a frame from a video. For me, it is a simple as clicking the Save Frame button.
I have a choice of saving an image in the PNG, JPEG/JPG or BMP file format. Any photo editing software can open these photo formats but I would suggest that you stick with either PNG or JPEG.
Once saved to your hard drive you can then use it in a photo editing application to re-size or add effects.
You should have some type of photo editing software on your computer or device. It generally is included as a basic feature of any traditional desktop or notebook.
If you don't have a photo or image editing software you could try to use one of the many web application to edit software such as Avary's Phoenix Photo Editor or Picnik.