>> I use my computer is tablet mode 90% of the time. <<

Think your best bet will be to create a desktop that matches your tablet
aspect ratio... i.e. an image in portrait orientation sized to match the
table screen.

That will size correctly in tablet orientation (portrait) then the
handling setting will determine how it displays in the landscape
orientation...

....suggest using tile or center if it's unacceptable when stretched...
might also look for an acceptable image that looks ok when distorted.

Beverly Howard