You store the picture you have on by uploading it to flicker or photobucket or somewhere on the internet that will give you an address to link to check the size...
if you upload to flicker there is a "see all sizes" option at the image top area so you do not link to a massively large photo and blow the forum up click on the medium size and scroll down select and copy to "grab the photos url" the copied
url link should look like this
http://farm1.static.flickr.com/189/535687387_c11fcd4915.jpg
Now in the reply to thread window here above the posting box where you write you will see the world and the chain link icon if you click that it will ask for the url - paste the copied url
over the blue highlighted http script in the box or you can select the mountain picture looking thing and do the same and your done the picture itself will show up in your post
hope that helps