i have an image i'd like to update, so i uploaded the newer version through ftp. but i noticed zenphoto resizes the images that goes into the albums directory, and i don't see anyway to upload a newer version other than doing it with ftp. is there a way to upload an updated photo through the admin section?

Sorry, there is no way except you delete the old image before you upload the new one. But with this all data like description will be lost (except you attach that as EXIF to the image itself). But maybe worth to think about this for future versions.

Zenphoto does not resize the image uploaded. It will however create a cached version sized for your theme. You can upload a new copy by the admin/upload interface. It will overwrite the original.