Double-clicking on Photoshop's background (the gray application background) is equivalent to choosing File » Open, or pressing Ctrl/Cmd+O.

The table below summarizes the various double-click actions as well as their equivalent commands and keyboard shortcuts:

Action Equiv. Command Equivalent Shortcut

Double-click File » Open Ctrl/Cmd+O
Ctrl/Cmd-double-click File » New Ctrl/Cmd+N
Alt/Opt-double-click File » Open As Ctrl/Cmd+Alt/Opt+Shift+O
Shift-double-click File » Open Adobe Bridge
Alt/Opt+Shift-double-click File » Save As Ctrl/Cmd+Shift+S

__________________
Cheers!!!