Okay, so here is a cheat that you can use.
1. Attach a pic to your post, and submit it.
2. Then copy the link of the recently upload image by right clicking it.
3. Now paste that link in these tags by editing your post [ img ] [ /img ] (remove extra spaces from tags)
And voila... you wouldn't have to upload pic to another server.
Here is an example:
Trying to post pic in message body