I would first try and add a new server just as file store behind those web servers, the web servers would upload the file to this server and proxy requests to i

I would first try and add a new server just as file store behind those web servers, the web servers would upload the file to this server and proxy requests to it (using a cache on the web server to speed up repeated requests). In case we can't add any new servers, I would save the initial web server address (ex: lan ip) where the file was uploaded, to the database (we are using a common database, right?) and when the image is requested (and doesn't exist on the current web server), reverse proxy from the initial web server. Those architectures are a little hack 'n' slash, but you got to do what you got to do.