Many programs can take a still frame and save as a bitmap file. VLC is good, I use MPEG Video Wizard.
You'll also need a good image processing program such as Photoshop or Paint Shop Pro to resize and perhaps sharpen the images.
Resizing is necessary if you're going to display the image on a computer screen or print on paper. Television aspect ratios are not the same as for computers, see this discussion which is probably more than you ever wanted to know about aspect ratios.
http://lipas.uwasa.fi/~f76998/video/conversion/
For example, if you captured a still frame from a PAL 4:3 source, you would need to adjust the horizontal dimension of the image by 109% while leaving the vertical dimension alone. Otherwise, the image will look distorted (circles will appear as ellipses, etc).
PAL 4:3 109%
PAL 16:9 145%
NTSC 4:3 91%
NTSC 16:9 121%