This is a topic that has been exercised before, for example here: Image resolution in Sketchup
Applying OpenGL maximum texture size does help a lot but at the expense of slower operation. I believe some cannot take advantage of that at all because of their graphics card, so it is not a great solution. Also, I have just noticed that when you transfer to Layout, all the resolution acquired gets lost again. So for output purposes it is nigh on useless. Unless I have missed something - always possible with me.
It seems to me that improving this would help enormously when importing, say, DWG files as you can import all the text that SU cannot normally cope with. By importing the vector info in the normal way and using an image file as an underlay, you could have the best of both worlds. But you do have to be able to print out the result as sharply as you see it in SU.
I don’t know how, or even if, all this can be achieved. But it would defo be high on my wish list for SU 2018 or a plugin.