i dont know
this might be a noob idea
but
if there are big blocks and the cursor is not small
then you could destroy the bg blocks that are the same size as the cursor when you touch the bg
remember i mean that that the bg will be in small blocks that are on the top layer
or you could:
have different pieces of the smiley face as different sprites, set there places next to each other like a puzzle then, set their z order to the bottom
then when the player touches the right places
mouse on click/hover over smileyfacepiece set z order to the top.
remember even though the smileypieces are not visible from the top a mouse hovering over them whilst they are under another object will still be detected.
yea so this would work if the cursor is big-if it is small then it will take ages to make-you could try making the smiley face into 4 pieces or less or more
so tell me if this is what you want
but i bet there is a better way
Radkampfwagen