Thumbnail gallery-how to center load images- I am stuck!

I have built a gallery similar to the one found on Kirupa with a siding thumbnail gallery that references xml. Everything works great except that the images I want to load at their large size vary in width so when they load referencing the upper left hand corner they are not centered. I would like to load them so that each image in centered in the “picture” image area. I created a new movie clip and gave it the instance name of “picture” and center justified it but the images now load with THEIR upper left corner in the center of the “picture” image area. do I have to add code to the xml? is there anything I can ad to th AS to load pictures centered? I don’t understand the code well enough to play around any more than I have. any help would be greatly appreciated as i am stuck!
thank you,