You need to have enough reputation to be able to insert a picture in your post: that privilege comes at 10 reputation points. A single upvote on an answer, or two on a question, will bring you to this point; the restriction is solely there to prevent drive-by spammers from posting images.
To insert an image in your post, click the img button in the editor toolbar or press Ctrl+G, then enter a link to the picture on the Web or the location of the picture file on your computer. The image will be uploaded to Stack Exchange's servers, so that it's always available.