Collision boxes

Is there anyway to change collision positions? Here is a picture of the player and a rock

http://www.madladdesigns.co.uk/mmf2/playercollision.gif

http://www.madladdesigns.co.uk/mmf2/rockcollision.gif

At the moment the collision takes place around the entire player and the entire set of rocks, i'd like the collision box to be where the dotted boxes are on these images (otherwise the player's head collides with the bottom smallest rock!). How would I achieve this?

Thanks yall :)

You can use separate invisible objects to handle collisions (a way to set them up could be: "Always set the collider object to (X, Y) from Player", and "Start of level: create collider at (0, 0) from Rocks".