I am at this very moment at Seymour Flux myself (second time through the game), my first time trying he must have killed me 20 times untill I discovered the best way to kill him:
1. Make sure all of your Aeon's and party members' overdrives are at max
2. Get Zombie ward for all of your characters (use Riku's customize - 30 holy waters on armor will do it)
3. Make sure you have at least one character who has more than 4000HP so somebody will live through anilhate
As for your Avatar: go to "My Profile" then modify your profile and in the Avitar section at the bottom you can input a custom web address (I believe you must have your own web space), upload your avitar to your webspace and enter the URL