Simply open the image in Preview (should already be your default image viewer in OS X) - see the screen capture below of an Alexander Calder lithograph from the web - these are saved as *.PNG files on the desktop (first pic below, blue arrow) - once opened, hold down the option key and click on 'File' in Preview's top menu bar - a new selection appears, i.e. 'Save As' - click on that choice which will open the dialog box in the second image - pick JPEG in the list (blue arrow). Dave
P.S. Vansmith beat me to the draw w/ yet another and quicker way to change the image format; agree w/ his suggestion to simply keep the PNG extension.