1. A computer-implemented method for verifying the authenticity of graphical images, at least a portion of the method being performed by a computing device comprising at least one processor, the method comprising: identifying a graphical image intended for presentation by a display; prior to facilitating presentation of the graphical image by the display: identifying an original unique identifier of at least a portion of the graphical image that has been steganographically encoded into one or more least significant bits of one or more pixels included in the graphical image; computing a subsequent unique identifier of the portion of the graphical image; and determining, by comparing the subsequent unique identifier to the original unique identifier, whether the graphical image is authentic.