SOS! How to Know What File is HAUNTING My PC??

:cry SOS! My PC is haunted by a virus, trojan or some sort of malware and I'm afraid it's going to start gobbling up everything on my hard drive.
Here is a MAJOR IRRITATION that seems it could be part of a BIG PROBLEM: When I completely shut down the computer and the Microsoft Windows splash screen appears -- just before the screen SHOULD go black -- my entire screen shows what appears to be a 1950s photograph of a LITTLE GIRL -- just for a second or two -- then the screen goes black and it shuts down. The pic takes up the ENTIRE SCREEN and is only there for a couple of seconds. I have NEVER seen this pic until other problems began several weeks ago and don't know HOW or WHAT culprit to look for in my files.
The pic is NOT one of mine and I am the ONLY person who uses the computer. I have looked extensively through all the folders I can find that include pictures, but due to having several different tekkies work on my 'puter over the past two years and each leaving it the way they thought it should be left, I now find all sorts of REALLY OLD stuff, including pics and docs, that I thought were long gone or programs that they must have downloaded, but which are not in my Add/Remove on the control panel. I have absolutely no idea which of them are necessary and which are just leftover junk.
I'm nowhere NEAR being a MG, but I DO KNOW that this WEIRD LITTLE GIRL'S PIC should NOT take over my screen as I try to shut down. ANY ADVICE on how to FIND the file or pic or whatever it is would be MOST APPRECIATED!!