I remember a bit of discussion about this previously, but couldn't find it again.

I was wondering if anyone could use this bit of code from an OSCommerce contribution to use in ZenPhoto?
http://addons.oscommerce.com/info/5555

"This is a small contribution which simply verifies that the width and height of a newly uploaded image is within acceptable boundries.

If not, the image is resized to within the boundries, maintaining the aspect ratio using simple calculations, and resaved over the uploaded file.

This will not expand images which are below the boundries due to pixelation which will occur upon expansion of small images."

We have already an admin option called "allow upscale" (or the like) that does actually the same. If not checked no smaller image will be resized.
Also Zenphoto does not resize on upload, it resizes when someone visits your gallery using the option settings or special theme settings or if you precache.