I would never bother to scream about such small stuff, especially when the intentions behind the suggestion are helpful. But, sadly, to no avail: I haven't set anything to stretch small pictures, and this slider that you mention--of which others before you have spoken--does not appear in the settings option of my (Windows 7) screensaver (unless, of course, it is cleverly concealed someplace just to make the average computer user's life more challenging). In any case, I've grown quite fond of the Mystify light show, and I was getting sick of the photos, anyway. At least the pretty colored lights are ever-changing and seemingly infinitely variable.
Thank you, Andy, for this rare nonwithering response. I shall treasure it.