Upload the image you would like to use in your post to an image hoster (TinyPic, ImageShack etc).
Copy/paste the URL of the image inside tags, like this:
[img]URL of image here
Hope this helps you!!
Alternatively, you can just attach the image to your post via Manage Attachemtns. Find the image you want and upload it to the PC Server.
ADDITION: You will have to have 15 posts to upload images using IMG tags. However, there are no restrictions on using Manage Attachments for images :)