Ok, well, here goes.
If you are using paint, then hit IMAGE, ATTRIBUTES. Tick the box (inder UNITS) where it says PIXELS. Type in 80 in either the length or width, whichever has the higher value. Adjust the other way (either length or width) accordingly to keep the pig-ture proportional.
If you are using Adobe, just click IMAGE, RESIZE, and IMAGE SIZE. Use the pulldown menus to make the units pixels. Check the box on the bottom that says "constrain proportions". Then, change the largest value to 80, and the other will automatically be changed to be proprotional.
Those are the more common ones, so I hope that can help you. =)