This wallpaper has been created from the famous Tux vs MSN image.
Edited and modified with Gimp.
Download from here the image "Tux vs windows 7 fish.png"
Search in Google Images the original win 7 beta fish wallpaper(1920x1200) with the words: windows 7 fish
Download and then open with GIMP the original wallpaper (File > Open).
Then open the image "Tux vs windows 7 fish.png" as a new layer (File > Open as Layers)
You could now export the image as .jpg under your own responsibility (File > Export as, and choose a name with .jpg extension)
*The preview shows as shown in GIMP.