To delete an image and its transition effect:

1. Choose Change Slides… from the Tools menu.
2. Click on the image thumbnail in the Change Slides dialog.
3. Click the Delete Slide button.

To delete several images and transition effects:

1. Choose Change Slides… from the Tools menu.
2. Select the images you want to delete. You can drag across adjacent images, use shift-click to select adjacent images, or use command-click to select non-adjacent images.
3. Click the Delete Slides button.

You cannot delete all the slides.