Hehe, I like the Gradius reference.
But I think your piece needs a bit more work before you can call it finished. Here a few things I've noticed.
For one, the Moai needs volume. The weird light-edges-at-top thing needs a reworking. It looks like selective outlining gone wrong. If you're going to blend into a dark scenary, you need to anti-alias with darker buffer shades, not bright edges.
The shadow color is too washed out to really work, make the thing a richer purple or possibly go with a dark reddish-brown. Actually, the real problem is that bright orange. If you make it less saturated, the thing starts to look a lot better.
If you look at most Moai heads you can't really see the eyes, because they're obscured by shadows
http://upload.wikimedia.org/wikipedia/commons/a/a2/Moai_Rano_raraku.jpgNext the background, is that grasss, or rocks surrounding your Moai? I recommend adding a lighter and darker shade to better define volumes. Also, space background details. Gradius always had a heavily starry sky with planets flowing by.
Next, you need to make the Moai stand out from the background, while not clashing with it. So make him pop out from the other foreground elements
I wouldn't have realized that was the Vic Viper at all if it weren't for the title of the thread, because the view is too obscured. It's a nice silhouette but it needs something to make it more readable as a ship. If you look at the actual Graduius game sprites, the ship is still readable from underneath, because it has some volume to it. Yours looks like a flat red spiky cloud right now.
I'd probably say switching to a diving down view of the Vic Viper if you want it to be noticed.
Oh, oh, and where's the ship's blue flame trail?
EDIT: Realized a mistake for this piece. A darker purple shadow makes things worse.
EDIT 2: Made an edit to the palette and the Moai head:
EDIT 3: Doh, I didn't realize the palette was limited for this contest. FIXED.