Are you in Timeline mode? If so, you should see each photo as an individual clip on your timeline, so it should be pretty easy to select only one. (It might help to click someplace else on your timeline first to make sure you haven't already got the group selected.)
Of course, it also depends on how you created your slideshow. There are a number of ways to do so in Premiere Elements.
If you created your slideshow in the Organizer's Slideshow Creator, you will need to first right-click on the slideshow on your timeline and select the Break Apart option.
1) Slideshow created in Elements Organizer and then imported in Premiere Elements
Such a slideshow appears as a video file in PrE, you can apply Pan and Zoom tool directly to this file in Sceneline or Timeline, or you can Rightclick the slideshow in Timeline or Sceneline and select Break Apart Elements Organizer slideshow, and then apply Pan and zoom tool to individual photos.
2) Slideshow created in Premiere Elements itself
You cannot apply the Pan and Zoom tool to this entire Slideshow in timeline or sceneline unlike above. In timeline, you can select individual photos and click on the Pan and Zoom tool. In sceneline you can select the slideshow and right click and select Ungroup scenes and then select a single photo and then go to Pan and Zoom tool.