Assuming you built the slideshow in Premiere Elements, zoom in as close as possible to the spot on your timeline where the picture appears. (Zoom in with slider or by pressing the + key on your keyboard.)
You may also be able to see the phantom slide in Sceneline view.
I agree with Steve's suggestion to switch to the Timeline View Mode, and zoom in to the max. There is a slider above the Timeline, that will allow you to do a max zoom. I would hit the Home Key, to get to the beginning of the Timeline and then use the PageUp and PageDn keys to "step" through the Timeline, one Clip/Slide at a time. This process is for two reasons:
1.) you will be looking for gaps in your Video Track
2.) you will be looking for your "phantom" slide
I would increase the size of the the vertical on the Timeline Panel, to see all of the Video Tracks, as your "phantom" slide could be on another Video Track besides Video Track 1.
As you step through each Clip/Slide, notice the movement of the CTI (Current Time Indicator w/ the red Edit Line). It should jump from Clip to Clip. If it seems to hesitate, or "hiccup," there is either a gap, or something else there, causing it to hesitate. At that point, make sure to scroll the vertical on the Timeline, if you cannot see all Video Tracks, as a Clip (even a single Frame), might have gotten moved away from Video Track 1.
Good luck, and good hunting,
If you can't find it on your Timeline by Zooming in as suggested, check the other Video Tracks, it's always possible that the Photo jumped up (or down
to another Track, when you were moving things around).
That seems to happen to me all the time, especially when rearranging the order of Photos on a Timeline...
Thank you! You people who help so quickly have all helped to keep me sane.