I agree that its not the best picture to judge the dress by. It is very shiny, but when its actually on, I'm sure it looks better.
I'd be more concerned that being polyester, you will sweat terribly underneath. Do you have a cotton petticoat so the polyester isnt directly on you skin? I bought a cheap Chinese dress that was polyester. Just trying it on, I was literally dripping in sweat, I cannot imagine how it would go if I was dancing in it.
If you tried your dress on & you didnt sweat, then I'm sure yours has a better liner than mine did. Can you post a pic of you in the dress?