A handy tip when opening multiple photos into Photoshop out of tabs
In Photoshop you might be irritated to find that new windows automatically open in the same window as your open image (in a separate tab).
To compare images side by side, you have to drag the tab out of the window and onto the background. The solution is found in Edit>Preferences>Interface.
Uncheck the box that says Open Documents as Tabs and then restart Photoshop.