Today I thought about what it looks like when I see a nice motif on my Windows wallpaper. To do this, I simply drag the current background file from the folder /windows/web/wallpaper/windows/img0.jpg into the Picture-Designer. 

Then I load the image with the desired motif and open the image section dialog with double click. There I roughly define the relevant image area. As the next thing I open the "Create Free Filter" dialog click once in a circle around the exact part of the image, which will later be used as a motif. 

After this part is done, I position my subject on the Windows background area and leave some space in the lower area. The Windows Quick Launch bar appears there. So, now export the image as jpg in the format 1920x1200 and already the new background is available.

Now open the context menu (with the right mouse button) on the result image and click "Select as background image". Now, I have my new wallpaper active.