If you are playing back your slideshow in the PSE Slideshow Editor and you see a slide(s) that you need to edit, there is no need to exit the Editor.
1) Click on the image above the Timeline control buttons (NOT the image on the Timeline) and bounding bars will immediately appear
2) Click on More Editing at the lower-right of the screen in the Properties panel, which will bring up your image in the PSE Editor
3) Edit as necessary
4) Click Ctrl-Q to exit the editor
5) When asked if you wish to save the change, click Yes and--Voila!--your newly-edited image is now on the Slideshow Editor Timeline.
6) Save the new Timeline