You can set default attributes such as shadow, reflection, opacity, and border (stroke) color, so that any image you add to your slideshow automatically uses the default settings.
In the slide navigator, create a new slide.
If you’re setting up default attributes for a particular master slide (rather than all masters using the current theme), click Masters in the toolbar and choose the master slide.
Place an image on the slide and set its default attributes. For more information, see the following topics:
Do one of the following:
To make the image the default for only the current master slide, choose Format > Advanced > “Define Image for Current Master.”
To make the image the default for all master slides in the current theme, choose Format > Advanced > “Define Image for All Masters.”
If you don’t want the image on the slide, delete the image.