Just tried and using the sctoll wheel can make the photo fill the whole screen which you asked for in your original post…
Yes but it doesn’t stop the slideshow going through it’s motions @realspeed , and I would have to create a seperate file for each and every photo…
Good reason to go video then with 4K no problems then 
Too many other hobbies and interests realspeed… 
…Well! I lied about the juggling… 
I tried that on a 1920 x 1080 photo (the native resolution of the monitor) but there was still a border around it.
Put masking tape around the edge monitor screen seems to be the answer then
That’s not helpful realspeed… 