To discovery which file type you have to use for your image just follow these simple instructions in following priority order:
Have text? Use PNG.
Is a piece of art like a draw, a painting or a webcomic? Use PNG.
It is… moving! Use GIF.
Is a photo? Use JPG.
Is not exactly a photo but contains photos (like people. trees and landscapes)? Use JPG.
Is not a photo, does not contain a photo but I remain concerned about the size of my file despite the breakthrough in telecommunication speeds. Try PNG with indexed palette and Floyd–Steinberg color dithering.
Nah, man. Use JPG but with all lower compression or higher quality options you may find.
It’s nothing listed above! Sir, your problem is far away from the scope of these instructions.