How do I delete a picture......

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I wanted to response to a discussion, so I uploaded a picture, and I realized that it was a wrong picture. I wanted to delete it, but I didn't know how to, so I didn't post the response. Now whenever I try to reponse to another discussion, and try to add an image, that same picture shows up, and there's no option for me to delete that picture. Anyone know how to do it ???