Multiple Movie Clip Loaders

Hello All,
I have a question about movie clip loaders. Currently I am working on a portfolio website for myself and have a section in which the user will be able to click on a thumbnail of a flash banner and the actual banner will populate in the movie clip loader area defined on my stage. I want one banner on at a time and everytime a new thum is clicked I want that banner to replace the one currently playing. My main issue is that alot of my banners are different sizes so my first thought was to make one movie clip loader which was large enough to fit all of the banners. The problem with that is the banner populate in the top left corner of the movie clip loader hence the smaller banners load all the way on the left of the screen (i want my banners to load centered). Then, after failing to find a way to center the banners within the movie clip loader, i figured that i could create two or more movie clip loaders which works except that i cant get one movie clip loader to stop playing and clear the banner when the other movie clip loader is activated. Right now it looks like there are multiple banners on the screen at once…Im sure, in my limited knowledge of action script, that I am doing something fundamentally wrong–any assistance that a kind soul might offer would be well appreciated!!! Thanks!