lol u cant but u good take a screen shot of it then paste in paint and get the picture from there
http://www.bleepingcomputer.com/tutorials/tutorial96.html
Dirty or not the best way is to take the screen shot by pressing Print Screen on your keyboard. Open Paint then do Control V on the keyboard to paste the screen shot in your Paint program. Then outline what you want the .jpeg of and save as...
I would suggest you finding a way to open the picture with Microsoft Paint. Once you do that, you can then click on "save as", and that gives you the option to save it as jpg.