These are just simple suggestions to help you get the best wallpaper viewing experience out of your new Nexus 7.

1. For best viewing and crispness, wallpapers should be at least double the screens resolution. I find 3840x2400 is a sweet spot. while the nexus 7's resolution is 16:10, Android crops wallpaper
as if it were a 1:1 aspect ratio (i.e. a box, but minus the corners). This means even wallpaper matching the
resolution of the screen, will not fill the screen. your 1200
vertical becomes stretched along your 1920 horizontal, which isn't good, especially since Wallpaper is stretched out across 5 home screens and needs to move slightly when swiping between them. the cropping effectively means your
1920x1200 wallpaper, crops out to a resolution of essentially
1200x1200 only.
This means wallpaper will be blown up and blurry rather than crisp.

2. Try to use wallpapers with a lot of border space. what I mean by that is, wallpapers with most of what's going on in it, in the center. I tend to keep my icons along the sides of the screen for ease of access with fingers. even though I use a stylus, I still like to keep the center of my screen unobstructed so...
#A : I can actually see and enjoy my wallpaper And
#B : So I can more easily Identify the
icons and text on my home screen. it's much more effortless when you don't have to pick out icons through all the background noise, even if you already know where they are, you still take a bit longer to
identify/read them when they're somewhat blending into the background.

3. Darker wallpapers are easier on the eyes and better
for text readability, but
entirely black can be dull. although this isn't as much of a problem if you don't keep icons on your home screen or if you have the border
space mentioned in [2]. The easiest way to search for wallpapers is google's image search. under search tools, set the exact image size to search for as "3840x2400", while searching the word "wallpaper". you'll find plenty of Beautiful Wallpapers from many sources that will give you the best wallpaper experience possible.