They suggest you can put up your older vote incentives there but really anything goes. I use mostly chapter cover art and any comic related illustrations I've done recently, but I've also seen people put pages up as well.
You also need to re-enable the bonus incentives every month - they do this to keep the incentive gallery from becoming completely clogged up I think. They reset the bonus incentives a few hours before the actual votes reset, so I just go in and re-enable mine (or swap in new ones) right before I go to bed on the last day of every month.
I really wish we had a way to see where viewers are coming from, because I'm not actually sure how much TWC is beneficial. I definitely see an increase in views on my comic in months where the bonus incentives push me high on the list at the beginning of the month, but it's hard to tell how many, if any, of those views are becoming subscribers.