Thanks Eibro, this problem has been already solved - the same idea was provided by Magos, however, there are other architectural problems.
Besides, if the selected sprite was not the topmost, then it's Z-order must be changed, and all the others which were in front of it must go one level deeper in the Z-buffer - to avoid the unpleasant effect which can be seen here:
http://www.unitedmedia.com/comics/pe...le/puzzle.html
Unfortunately I have no time for my puzzle game right now, but I'll finish it this year I hope
