Currently, there is no method by which images can be utilized directly from Visualizer and NameMapping in comparing images on screen. This, I believe, was already mentioned above by @AlexKaras . This, I believe, answers your question: You will need to create a repository of baseline images to use in your comparisons so that you can either use the PictureObj.Difference method or a Region Checkpoint to compare images.
If you want such a thing added, there is a feature request area here on the community that you can request it. However, placing a request does not mean it willl get added/created/developed in the short term... meaning, you will STILL need to do the work to create that image respository.
Why automate? I do automated testing because there's only so much a human being can do and remain healthy. Sleep is a requirement. So, while people sleep, automation that I create does what I've described above in order to make sure that nothing gets past the final defense of the testing group. I love good food, good books, good friends, and good fun.