When you drag a selection out of ShowImage and onto the desktop, a new file containing that selection is created on the desktop. Most users figure out as much within about five minutes of playing with ShowImage in R4 (where this feature first appeared).
What is less known is that, by holding down the control key when you drop the clipping on the desktop, the Tracker will display a menu which allows you to choose the image file format to create the clipping in. If you would like a JPEG clipping (say, to put on your web page), just choose

image/jpeg

from the menu, and that’s what you’ll get!