An old post of mine (that probably gets more hits than any other on this site) needs a bit of an update. A few changes took place in recent years with 3ds Max, namely with Nitrous as a display driver, and that meant doing things a little differently to get the same effect. So, without further ado, I bring you "getting high-res textures in Max's Nitrous viewport". Note: this isn't how you change things for the better in a different type of display driver. For that, refer to the original tutorial.
So the banner image I tossed onto a plane looks cruddy and nothing like the magnificent banner you see above. The image isn't broken, nor is the viewport. It is just giving you a compressed preview of the actual image. Why? Well, it makes things run faster, of course. You don't need that, though. You've got a beast of a machine, right? No? Maybe you just want to see it as-it-is? In real life? Okay. Fix it.
In your viewport, click on the current view (shaded, in the example above) and click. At the bottom of the menu, select "Configure...".
In the Viewport Configuration panel that comes up, select the "Display Performance" tab.
This is where that hi-res magic happens. Just change the default "512" value (or whatever value it is) in the respective channel, and hit "Apply".We don't necessarily want to overload our viewport with hi-res everything, all the time. At times, be it with a reference image, a texture for a character model, or a procedural material shader, we want to look at all the goodness we are working on and preview things a little more clearly.
And there you go. Glorious.