Right click on the Desktop, select Personalize to change settings. Change the setting under Fit to fill the screen.

Tip--Click on Themes to see some fun options.

Yes, when first using my new Dell I loved the Collage screen saver.  When I set up the computer I simply chose My Pictures and worked great.  It filled the screen with half dozen photos of different sizes and was a great reminder of past people and experiences.  I went in and changed the timing and the whole thing went away.  I can't seem to get it back.  It must have been from Dell.  I have searched Microsoft and can't find anything like it.  "Personalize" gives lots of options but not that one.