How to load images in the center than top left?

Hello, I am a beginner in Flash, and I am creating a gallery with XML. I am trying to make the gallery load image in the center rather than the top left, and I am very stuck.

I have a movie clip layer named picture_area, and inside it has two layers. The top one is picture_mask, a mask of the size 700 x 400 (width x height), and the second layer is picture_load, just an empty movie clip to load pictures.

At first, everything works well because all my images is 700 x 400, however, I start to run into images that cannot be cropped at that size, they are maybe 500 x 400, or 700 x 200. Those images are loaded from the very top left, leaving a huge white space on the rest of the 700 x 400 area. I want to even it out by placing the images in the center.

Is there any way that I can make Flash to know the size of the image, and it will just place it in the center?

Thank you for your time. Looking forward to your replies.