Fade Transition effect on large photos

Hey guys, love the site btw, finally got too stumped on a question and had to ask tho.

Relatively simple one probably… basically, I’m an amateur, but I’ve been working on the image gallery on www.ctphotogroup.com and to do the fade transition between the images, basically I have them both loaded as movieclips, and I programmatically reduce the alpha on one while increasing the alpha on the other to do the transition. However, I’ve noticed on big photos the size they want, it really slows down the frame rate and looks choppy. There must be a better way to do it?

For example, just doing a quick search for photography websites I found one over at:
http://www.lesliegilbertphotography.com/

and he seems to transition between almost comparably sized photos much smoother? Is this an alpha mask blend (just found out about those today), or would that improve it?

Any info/direction would be greatly appreciated!

Thanks,
Dave