1) Reveal using an exit animation on shapes
Insert your picture on the slide.Draw a shape (rectangle in the example) and copy the rectangle to cover the whole of the picture.
Select the shapes in the order you want them to appear (shift+click). Add the exit animation 'Disappear' using the Animation tab and the Add animation button.
In the animation pane all the rectangles (except the first one) have an animation that
starts with previous. Select the second animation and drag the bar in the advanced timeline to add a delay of x seconds. Repeat that process for the following animations.
To make this a little easier zoom out on the advanced timeline, click on the list arrow next to seconds and choose zoom out.
Or drag the animation pane to the bottom of the screen so that it appears in a horizontal position.
After the first click each rectangle disappears with a delay till all the rectangles have gone.
2) Reveal using an emphasis effect on shapes
The procedure is identical to the one described above.Select the shapes in the order you want them to appear (shift+click). Add the emphasis animation 'Transparency' using the Animation tab and the Add animation button. Right click the selection and on the Effects tab of the dialog increase the default transparency percentage from 50% to 100%.
In the animation pane all the rectangles (except the first one) have an animation that
starts with previous. Select the second animation and drag the bar in the advanced timeline to add a delay of x seconds. Repeat that process for the following animations.
In the video you will see that there is no visible difference between the first video and this one.
3) Reveal temporarily by adding an exit animation followed by an entrance animation on shapes.
If you want to let people guess what/who the picture represents and make it a bit more difficult you can let the shapes disappear then reappear after a small delay.Select the shapes in the order you want them to appear (shift+click). Add the exit animation 'Disappear' using the Animation tab and the Add animation button, choose On click next to the start option.
Drag the entrance animation just beneath the correct exit animation.
Now select all the entrance animations (ctrl+klik) and increase the timing next to delay.
See the result in this video:
4) Reveal temporarily by adding an emphasis animation on shapes
Select the shapes in the order you want them to appear (shift+click). Add the emphasis animation 'Transparency' using the Animation tab and the Add animation button. Right click the selection and choose Effect options, on the Effects tab increase the default transparency percentage from 50% to 100%.Now click on the Timing tab in the dialog and next to start select On click. Next to Duration choose Until next click. No need to add a delay this time, the appearance and disappearance depends solely on you clicking.
5) Reveal step by step and yet show the entire picture before all the animations have finished.
You can add an extra bonus to method 3 and 4 by adding the possibility to show the entire picture at will during the slide show. To do so you need to apply and exit animation to all the rectangles activated by a trigger.After dragging the entrance animations to reach the right position. Select all the rectangles again and add the exit animation 'disappear'. In the animation pane all the animations are animated with previous. Leave the selection active and click next to the Trigger button and select On click of then choose the shape you want to use as a trigger. In the example below I have chosen rectangle 1.
