I suspect you are facing a permissions issue on the roboRIO. I believe that the Java robot program will run as the "lvuser" not as the "admin" user.
On the roboRIO, the admin user is similar to the root user and has the ability to write to or modify the entire file system.
Since your program is probably running as the lvuser, I suspect you can only write to the /home/lvuser directory on the roboRIO account (unless you log in as admin and change permissions on your /images directory).
Try opening "ftp://roborio-TEAMNUM-FRC.local/ in your web browser to verify that you see a /home/lvuser directory on the roboRIO. If so, try using "/home/lvuser" as the output directory for your image files instead of "/images". If that works, it is definitely a permissions issue.
You can either then live with using the "/home/lvuser" directory, or use a tool like Putty for Windows (
http://www.putty.org/) to ssh into your roboRIO and change the permissions (or ownership) on your /images directory) as indicated in an earlier post.