Tool Tip on movie clip

Sorry if this has been covered someplace else, but I have been searching and have found nothing so far.
I am making a photo gallery that loads pictures from a xml file. I have an empty movieclip on the stage that the photos are loaded into. I want a tool tip to pop up when the mouse is on the picture. I have all the code for the tool tip working(it works fine on any of my buttons or text boxes) but not on the movie clip. If I trace the height and width of the movie clip, it says undefined, so I tried adding content to the clip so that is it the same size as the pictures, but still did not work.

picture.onRollOver = function() {
[INDENT] captionFN(true, description[p], this);
[/INDENT] [INDENT] this.onRollOut = function() {
[/INDENT]  [INDENT]   [INDENT] captionFN(false);
[/INDENT]   };

picture is my movieclip. captionFN is just the function that turns the tool tip on and off. Any help would be greatly appreciated!