@Blastar - Scaling the tiles to match up with the "block shadow raster" is a smart idea, thanks a lot for suggesting it and for creating the mockup graphic.
I was racking my brain for several days thinking about how I could use pre-rendered shadows (since they are not moving and not animated) - without finding a solution.
But I think that your idea could be the way to go... scale down the tile size and swap out the tiles which have a shadow on it if needed - will post results as soon as possible

@Raz - I am glad you like it - I am very happy with the new visuals also, in comparison it has looked too flat before, now the game objects literally "pop out" of the screen.
Thank you for your additional suggestions:
- I will adjust the frame shadows of the background but I can't remove the right-side shadow completely because the (non-shaded) background green and the frame grey
don't provide a sufficient contrast to divide the play area from the outer frame properly - but I will make it much smaller (left and top shadow will be adjusted too).
- Yes, the missing transparent message box shadows were already on my todo list - but thanks for suggesting it nevertheless.