you pinch the home screen. it then gives two options 1. Wallpapers, 2. Widgets. Simply select whichever wallpaper you want and 'set as wallpaper'. Then your done. The auto-rotate function means the wallpaper can't be cropped, as you say. The only way that could be, is if your Tab is rooted and your not using the standard wallpapers. In which case you have to download appropriate wallpapers in the correct size picture.
If your Tab is rooted, you run all sorts of minor risks, some being that wallpapers won't fit, graphical glitches, crashes etc. Not to mention the fact your leaving your tab wide open for abuse from external sources. I would not recommend rooting for what it gives you in return.