So, I opened up the two photos I wanted to merge, placed them side by side and resized them to my preference . (Make sure your iPhoto preference for the "Double-Click" option is set to "Opens in separate window".) Next, I took a selective screenshot (Cmd+Shift+4) of the two pictures. Since the screenshot is saved as a .pdf file, you need to open it up in Preview and export it as a JPEG (File -> Export), or whatever you like. Finally, click on "Options" and set the Quality slider to "Best". Click "OK", then "Save" and you've got yourself a merged picture.

