How to make appear the category image if there is'nt the product image?

[Giovanni14]

Hello everyone and thank you in advance for your help.
My problem is with the product images. If there is no product image VM gives us the opportunity to see a generic image that warns of the absence of the image (usually: noimage.gif). Now, instead of this image I would like to make appear the image of the category to which the product belongs. How can I do?
thanks
Giovanni