You do not need photo evidence to receive an authentic grade for your signature. PSA/DNA relies on 50-100x magnification to determine whether a signature was printed or written (i.e., written ink will have strokes of different thickness where two lines crossed paths such as a “t”). They also have historical documentation of known autographs from the signer to compare each signature, including the styling of the writing and the pressure exerted. Part of this process can be seen in “real time” on the History Channel’s Pawn Stars, where Steve Grad (previous PSA/DNA lead authenticator, now at BGS) provides an expert opinion on various pieces.
According to PSA, they label these techniques like this:
PSA authenticators conduct ink analysis, autograph structure analysis, object evaluation and side-by-side comparisons. Authenticators may also use a video spectral comparator to further evaluate the autograph.