Actually, I disagree....tubes, as in PSP tubes (.tub, .PSPTube), are used in Paint Shop Pro, they are to be "plugged in" (installed) into the correct folder to be used in that image editing program to function as a PSP tube.

.PNGs can be used/opened by any image editing program (PS, CS, PSP, Gimp, etc.) for 2D purposes such as website designs, signatures, printable cards, and various other decorative ways. 

Most look at these "tubes" for 2D purposes, especially figure "tubes".