Galaxy Tab S6 - How display image full screen

Resolution 1600 x 2560 pixels, 16:10 ratio.
That is the information directly from your device display...
You would need to make an image file the exact pixel dimensions of 1600 x 2560

 

That and go into your favourite gallery app, select the image, and if necessary tap on it to remove any toolbars (most image viewers will initially show it without, but in general tapping on the image being displayed toggles tool bars on/off).

That's all that's needed.