Here are the versions of @Jochen 's winning image for April. Offered in 4 versions:
4.3 ratio and 16.9 ratio calender desktop pages (PC) File sizes: 2.47 MB / 3.08 MB
4.3 ratio and 16.9 ratio calender desktop pages (Mac) File sizes: 2.52 MB / 3.10 MB
(Note, if you open the image, it will expand to it's full size and take ages to open... so follow the below instructions.)
Right click on image - Save image as - download. Open image - right click on image - Set as desktop background. Voila.
(Please let me know if you have any problems downloading.)