1. A system for authorization and control of an Electronic Billboard system operated at a location remote from a video program origination location comprising: a user key, presupplied to the Electronic Billboard system; a broadcast key, supplied to the Electronic Billboard system at or near a time of transmission of a video program to the Electronic Billboard system; and, means operated in conjunction with the Electronic Billboard system for selectively enabling the Electronic Billboard system to operate on said video program to effect a substitution of a chosen indicia into a preselected portion of image existing in said video program upon said user key and said broadcast key being found to correspond to a predetermined criteria, wherein said means for selectively enabling is implemented as a lock which operates to disable operation of the Electronic Billboard system unless said user key and said broadcast key conforming to said predetermined criteria are present.