This may have something to do with the Stretch setting you selected when you inserted the image into the slideshow. The four options available to you
are listed below. As you see, if this is not set correctly your image may not resize as expected to different devices.
The content will preserve its original size.
The image is resized to fill the destination dimensions. The aspect ratio is not preserved.
The image is resized to fit in the destination dimensions while preserving its native aspect ratio.
Uniform to Fill:
The content is resized to fill the destination dimensions while preserving its native aspect ratio. If the aspect ratio of the destination rectangle differs from the source, the source content is clipped to fit in the destination dimensions.