Right click on the photo you wish to crop, then select the choice of editing the photo externally.  You may need at this point to download the Photo Designer 6 tool (free download) that accompanies the Photo application you are using.  If you have it already downloaded, Photo Designer 6 will open up with your photo displayed.  Look to the left vertical tool bar and select the crop tool.  When you click on it you will be able to draw crop handles on your photo by left clicking on it and holding the left click down while you move it across your photo.  You can expand or contract the crop handles.  When you are finished setting the crop handles, just double click and the crop will occur.  Then you have a choice of having the newly cropped photo replace the original and if you select that, when you exit out of the Photo app, you can see the change occur in your slideshow.