Why do you need to know how to post a picture this earlier when there are other vital things you need to learn about the forum and the bitcoin currency itself, or maybe am missing something out here that you wanted to open a signature campaign which requires you to post an image, irrespective of your need for that, here are the important links to help you through knowing how to post an image.

guidelines for posting images
https://bitcointalk.org/index.php?topic=3974517

[Tutorial] How to add image, resize image and make image clickable
https://bitcointalk.org/index.php?topic=2958771