FTP reupload or update (overwrite images)

Hello,
I have a gallery site almost ready for production. I have a collection of images whose metadata gets updated frequently. Therefore, I need to be able to re-upload images that are already in the Gallery database. I understand that I could use the reupload module to manage the files individually. However, my website is interfacing with an external program that modifies the image files automatically. I don't want to manually reupload every file every time it changes. I am hoping someone knows a way to tell gallery to scan a folder on the server and automatically update all files that are new (or) have been changed. The important concept here is that I want to replace (overwrite) files that have already been uploaded (or rather, I do not want to duplicate images that already exist).

If the above is not possible, is there a way to delete items from the database based on the filename? I would not mind executing a mysql query that deletes all the items I am about to upload.

One last question is: is it possible to trigger a recursive import of images from the command line (shell)?

Thank you for any help you can provide!
-Nick