I want to model based on a picture.
Currently I can map an image onto a plane but this displays as a highly pixelated image despite the image being very high res.
Is there a way to display a sharp image in 3ds viewport?
for what you are doing, you should use viewport background..
select your viewport, then from the View menu on the top, go to viewport background, and select a background image
on this dialog you will also probably want to check "Match Bitmap" in Aspect Ratio group, and "Lock Zoom/Pan" somewhere on the right, or else you won't be able to zoom into your high-res-image
beware that you may sometimes notice that your objects have suddenly grown or shrinked over the image, that is due to background view transformations, which may happen during zooming and stuff (i'm not sure if i have already understood when does this happen =)
to fix the problem, move again to the View menu, and select Reset Background Transform..
if this comes to you often, you can create a shortcut for it.
if after resetting the background transform your objects still do not match the image, then compensate the zoom on the viewport, and reset the background again. if you try it, you'll understand what i mean..
for example, if your image is larger than your objects, zoom in on your objects so they will become the size they should.. of course this will make the background zoom together with it. but that's when you reset the background transform, and it goes into place.
do not get scared with all that i have just said, i'm just preparing you for the problems that may urge to you while you are using a background image with locked zoom/pan =)