I recently bought a "Samsung Galaxy Nexus gt-19250m". So far I'm enjoying the phone, but one of the things that drove me crazy was trying to figure out the proper size for the wallpaper image. Allow me to save you that burden.
The Nexus's physical screen is 720 pixels wide by 1280 pixels high. Only half of the wallpaper's width is displayed at a time, whereas the entire height is displayed. Grinding the math gives an optimal wallpaper size of (720 * 2) x 1280, or 1440 x 1280.
Unfortunately, it's not as easy as that. Although the Nexus screen is, in fact, 1280 pixels high, the top 98 pixels are covered by the notification area and the bottom 48 pixels are covered by the soft keys. Griding that math again gives us 1280 - (98 + 48) or 1134 pixels.
Long story short:
To make a wallpaper image that won't get cropped or stretched, you need to create a 1440 x 1280 pixel image, with the actual visible part of the image is 1440 x 1134 pixels, and shifted down 98 pixels from the top.