There is quite a long thread somewhere about this. (But I cannot remember the link, I think it was about Not square windows, but this cropped up in it)
What you do, is you "grab" the bit of the desktop that will be behind the window, then you make a normal window the size of your image, without borders, with a canvas that covers it all, then you "paste" the desktop segment onto the canvas, then you "draw" your "little animated image", with alpha
You move the window round as you would want to draw your image, ie if the image moves to the left, it really stays still in the canvas, but you move the window to the left etc
What you do, is you "grab" the bit of the desktop that will be behind the window, then you make a normal window the size of your image, without borders, with a canvas that covers it all, then you "paste" the desktop segment onto the canvas, then you "draw" your "little animated image", with alpha
You move the window round as you would want to draw your image, ie if the image moves to the left, it really stays still in the canvas, but you move the window to the left etc