Go right ahead! I believe quite a few people use icons from other games...even their music. Especially Final Fantasy-themed games.
As Alexander said, it's plenty ok. You're not selling it, so they don't give a crap. If you sell it for, say 20$ (assuming it's somehow that good) and it gets famous, then and only then will they care, since they would be very eager to take a percentage of you profit...assuming, as I said, there is profit. They wouldn't give a crap about 1000$ even, they'd care about millions.