But even with the jpg on the desktop if you want to edit it you still have to open the file.
In fact it's harder that way because you have to minimize the screen you're at currently and then open the jpg off your desktop, edit it, then re-maximize the previous screen.
On PC you just open paint, pop in the picture, crop it, save it, done.
But if you can crop it without opening a program on a Mac, then kudos to mac for saving a step I guess.