Get the url of the imagefile and simply add an img tag around that url like this:
[img] www.mypicture.com/mypicture.jpg [/img]
to get the url of an image right click on it and choose 'properties' (ie) or 'image info'/'copy image adress' (firefox) or 'copy image adress' (safari). Hope that helps - I choose imageshack.us to upload files usually.