The Galaxy Tab has a native screen resolution of 1024 x 600, and to have a wallpaper size that is correct for both orientations requires 1024 pixels wide x 1200 pixels high.
This Tabloid is different to previous Android devices in that the home screen autorotates even though there is no physical keyboard (like the G1).
Also the home screen uses the left hand side of the wallpaper by default. With this in mind I've created a new wallpaper template for the Galaxy Tab which is in Photoshop format. Get it here.
Also I've created some wallpapers which you can get / view www.fire-horse.co.uk/Galaxy_Tab_Wallpapers/index.html. Just so you know, not all the wallpapers are 100% safe for work ;-)
PS Just like setting wallpapers on other Android devices, the built in tool does weird stuff, so you'll need the free App from the Marketplace called "Wallpaper Set & Save".
More Galaxy Tab fun here.