You are correct. The shorter .jpg extension was made necessary because MS-DOS and early editions of Windows had a three-character limit on extensions. .tif and .tiff are also interchangeable for the same reason.

Since small file sizes in PNG depend on the number of colors used, those using that form of compression may want to review the color scheme generators linked here: http://www.cartographersguild.com/showthread.php?t=567

Reducing your image to a few colors can greatly improve compressibility, but it makes choosing the right colors more important. Those of us who are stronger technicians than artists can use some help with that.