Choosing a file format for screenshots isn't just a guessing game, as tech blogger Amit Agarwal knows all too well. Luckily, he's decided to share his well-tested knowledge on which file types fit which types of shots. If you're trying to grab still shots of a video, Flash animation, or high-resolution wallpaper:
... Always go with JPG instead of PNG because the image file size would be smaller without much degradation in the quality ... To give you example, this Wall Strip video on YouTube would take around 92kb when saved in PNG format but that would fall to 20kb if we changed the format from PNG to JPG. Surprisingly, there isn't any remarkable difference in quality.
Hit the link below for more tips on choosing a space-efficient, quality-conscious format for your screens.