ImageJ has a lot of keyboard shortcuts.
Listing all keyboard shortcuts
For a verbose list of keyboard shortcuts, call Plugins>Shortcuts>List shortcuts...
The Command Launcher
Use Control+L (on Mac, Command+L) to open the Command Launcher, to call menu entries by name.
To open a list of some recent commands, and a list of the most frequently called commands, hit Control+9 (on Mac, Command+9).
To open new images, type Control+O (on Mac, Command+O).