Merging Two pics into one JPEG.. HELP me computer geeks!

Hello fellow bloggers. I have been trying to figure out how to get two pictures into one jpeg.  I see it all the time especially on tumblr but just can't seem to do it.  I have a MacBook Pro and I don't have Photoshop (too expensive).  Does anyone have any advice?  I really want to use it so when I post two book covers they line up correctly here on blogger.  Thanks for any help you can give me!