I was looking for that answer, too and found it by accident. When you insert a photo to your post, in the insert box, look at the bottom and select “Make this featured photo” (going from memory, something like that.) If your pic is already inserted, then when you are editing your post, look to the right column and there should be something saying “Make this featured picture.” Hope that helps. I would love to see the smile on your face when you finally see your pic in the Slider!
