I tried to delete phtos from my ipad by opening phto app and tapping photo stream. Tehn I tapped a photo and tapped again, but there was no delete photo button. How do I delete a bad photo from my photostream?

How do I delete single photos that I don't want on my ipad? I went to the Apple website and followed the directions. I tried to delete a photo from my ipad photo stream by opening the photo app and tapping onphotostream. Then I tapped a bad photo, tapped again, but no delete button showed up. What else can I try?