Movie clip and button instance error

[COLOR=#191970]:h: Hello there. I constantly meet the same problem by making my thumbnail gallery.
What I want to do is to create an enlarged preview of my picture by passing my mouse on its own thumbnail like a tooltip but for image.
So I import from library an image (thumbnailed) convert it to movie clip and in the Mclip stage I use action script with on rollover action. But I always get the same error : on rollover only for button instance. Do I have then to convert my image as button ???
I am lost . Please help me, I’m getting nuts. : (
THANKS !!![/COLOR]