For anyone who is still trying, I found a simple solution!
I am a long time user of photo gallery and have a kind of love/hate relationship with it. I recently discovered this issue after upgrading from windows 7 to 10.
It took me about 30 minutes to find this solution, so unless this is not working for others and just for me, shame on you Microsoft (sorry, couldn't help myself)
So.... here goes:
- Locate the Photo Gallery .exe file, in my case and most likely for anyone else:
C:\Program Files (x86)\Windows Live\Photo Gallery\WLXPhotoGallery.exe
- right click and select properties.
- open compatibility tab
- put a tick-mark in "disable display scaling on high DPI settings"
- click apply
- close properties
Now startup Photo Gallery and try a slideshow.
Please let me know if it worked for you, as it did for me.