Millisecond Forums

I have a problem that I cannot seem to solve myself. This is what I would like to do:

I present 9 pictures at once using the stimulustimes attribute. Then I defined 9 white pictures at the same locations. Now I would like the respective white picture to cover the original picture whenever a participant clicks on one of the 9 original pictures. These white pictures should remain on the screen until the participant has clicked on 4 of the original 9 pictures in total.

What I struggle with is the branching (?) code to tell Inquisit "if mouseclick on picture x then show picture y". I imagine that with the /erase = false attribute I could achieve that the newly presented pictures will not go away.

Any ideas on how to do this? Thanks in advance.