Consider this scenario:
A system running Windows 7, Windows 8.0 or Windows 8.1
Screen display set to a higher DPI setting; for example, 150% (Larger) or higher
Windows Essentials 2012 installed
Viewing pictures in Photo Gallery as a slide show
In this scenario, you may notice that when setting the display DPI to a higher setting, such as 150% (Larger) or higher, the slide show only uses a portion of the screen, rather than full screen.
This is a known issue with Windows Essentials 2012 Photo Gallery.
To work around this issue, lower the display setting to 125% (Medium) DPI or less. You may also open the pictures in another photo application, such as Windows Photo Viewer, to view the pictures as a slideshow.
For more information about Windows Essentials 2012, visit this page.