First of all, do you know any networking programming with Java? if not then you may want to take a look at this:

Introduction to Web Server Application

To just have some glance of what is networking like in Java. Once you're done, then here are the few steps that you'll need to do in order to do what you're trying to do:
1. Make a connection between your computer and server
2. Depending on how do you want to implement this, but I'd say if you want to have both copies on the local and server, then you can generates the image in the local computer, and then read it, then write it to the server by using the connection.

I personally haven't done the writing part, but on the web server example is basically writing the image FROM the server to the browser's client, you can do the reverse as well...

I hope that helps, you may also want to show us some code

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