No worries! If you have not selected a helper, or received a request from a helper you can simply remove the ad, take new photos including the new items, update the description, update the reward and publish a new ad.
If you have already selected a helper you wont be allowed to add more items to the original ad. This is because the helper applied for the original ad and may only be able to help you with those specific items. All you need to do then is create a new ad with the new items, select the same helper if you get a request, or choose another helper if the first helper is unable to help you with the new ad.