the simple OSX solution...
 

... is to open the file to fill the screen, then press shift-"applekey"-3 at the same time.

A screenshot will be taken. My computer saves it as a pdf file for some reason, but you can open it ans save it as a jpeg.

Pretty simple -- took me a month to figure it out.

Thanks for all your help, tho --

this vid was great, thanks for posting :D