If the image is currently only on your computer, not online, I know of two ways to have it show up in the body of a post:

1. Upload it to a photo-sharing site like Photobucket or Flickr, then link to it in your post. The photo-sharing sites that I'm familiar with, even offer you links in several formats for each of your pictures -- <img ...> (HTML), [img] (BBCode), etc. The BBcode ones can be copy-pasted right into a post where you want the picture.

2. Create an album at your profile, upload the picture to it, and link to it. The albums at PC also provide a BBCode link under each picture, when you're looking at your own album.