I have CS 4 and 5, and they allow me to open images side by side.

The default has changed - images now open in tabs that take up the whole workspace - but you can change that and go back to the old way.

Try right-clicking on the image tab, and choose "move to new window."

I believe there's a way to change the settings so it defaults to new windows for each document, but I kinda got to like the tab setup, so I didn't do it.