My understanding is that you can correct anything, but different rules apply once you have started to receive bids on an item. If you are changing photos, it would be worth adding a note to the item description to explain this.
My only experiences of this have been a) when I was bidding on something (not selling) - when the seller amended the description, eBay sent me an email to say that the buyer had amended the description and "you might want to take a look at the item to see if this changes your mind about bidding for it". ... and b) I have added new photos whilst selling, if someone's asked for a closeup or a different view of the item.