The image plane cannot be selected as an object from the window it is associated with. You may be able to select the front plane from the perspective view, but many workflows will turn "Look through Camera" on, so you'll only be able to see the plane from one view.
What I do is routinely name the image planes, and I name them consistently - "frontPlane", "sidePlane", etc. If you know the name of the image plane you can select it by typing in the command line, i.e., "select -r frontPlane". (no quotes, of course)
Once you've done that, you can highlight the command from the script editor and drag/drop it onto the shelf so you can have an image plane selection button.
You can also make buttons to turn them on or off - i.e., "setAttr frontPlane.displayMode 0", etc. To learn the commands just watch as you turn things on and off in the script editor.
If you just want to select them from the interface, you have to go to component mode and make sure the "?" selection mask is enabled, then you can select the image plane from any window.
