First, you upload the picture of the item you want to use into your photo gallery. (Most photos used in my stories are from Google images).
Secondly, you go into your story and click items. Click create new item. Write out the items description and click Picture. Click the name of the picture you uploaded. (The number of uses is basically how much times your item can be used, though I think you already knew that).
You should have already created the page you want your item to be found on so you click 'Page found on' and scroll down until you see the page. When done, click save Item.
If this was too hard to understand, click 'Help & Info' on the left of the screen. Right under 'My Stuff'. There are plenty of very helpful tips there written by the more advanced members of the site.