You'll first need to upload the image to a free image hosting service as our forums don't offer image uploading. Imageshack.us
is an option, for instance, or there is the Taleworlds Forum-specific TWim
that is run by a community member specifically for use with our forum (see thread
Then, just post the image file URL that the upload site gives you inbetween [img] [/img] tags. Most free image hosting sites have a "forum link" URL that they provide automatically.