Problem with onEnterFrame or similar

Hi people!
I’ve had some trouble making a preloader for external .jpg files. The problem is in the onEnterFrame, I’ll show you some part:

loadjpg = function(mc,jpg) {
mc.onEnterFrame = function() {
trace("lele");
 }
 } 

that is just a part of the hole function, but I think it will be enough. It traces “lele” once, not when mc enters a new frame. I’ve also tried to call it with different ways like: [mc].onEnterFrame, with (mc) { onEnterFrame() . Does anyone know how it should really be?