Stamp provides custom made stamp designs that can be used in your presentation. You can also create your own design for the stamp. This can be found in the empower section.
If you want to upload a stamp, click on Stamp option. A list of available stamps are displayed. Select the desired one and it can be inserted either in the selected slide or the whole presentation (1). You can also create your own stamp design, by just editing the text (2).
The stamp can also be removed by clicking on Remove option. This displays a window asking you to confirm if all the stamps from the presentation is to be removed.