JPEG? PNG?

The term “JPEG” is an acronym for Joint Photographic Experts Group, which is a method of compressing the data in an image file (“lossy compression”). The .jpeg image format is most common with photographs and digital images. 

For your shorthand: jpegs are great for multi-colored photos and pictures. 

On the other hand, a PNG file (portable Network graphic). A PNG file allows transparency.

Again, super simple: JPEGs are great for pictures and PNGs are great for files that require some transparency.