image replacement

Hi
I’m currently changing the skin on my wordpress to one that uses a different size image.

How can I make all current images default to a picture of my choosing. (Which will be the correct size) If I had access to the database I could paste a new image name in the right place.

Is there an alternative way to change the image?

I understand that if you don’t use an image some templates can use a default one.

I intend to add new images of the correct size as I add content. So all previous images need to default to one default image.