The others seem to be ignoring your question for the most part, so I was simply trying to help in a way that would hopefully be more productive. An overlay is one partly transparent photo superimposed on another. This allows the position (if the case may be) or shape to be compared in a manner much easier than simple looking at a photo. Is this what has you confused?
su·per·im·pose ˌso͞op(ə)rəmˈpōz/ verb past tense: superimposed; past participle: superimposed place or lay (one thing) over another, typically so that both are still evident. "the number will appear on the screen, superimposed on a flashing button
Yeah, no kidding. Now try to answer the question instead of defining what I already know. Can you do one or not?