Its because when using the button we have for it when posting, you need to have it uploaded to a site
However, further down the list of sections on this forum, there's a section called "Member's Aquarium and Fish Pictures". Here you have an option to upload from your computer to this site, and then you can link in other sections from there on Good luck
--Edit--
When in that section of the forum, click "New Topic", then scroll down and you'll have a bit saying "file attachments" here you can select the folder its in on your computer and then click ok or add, or whatever it says
I use 250free.com It is a free site to upload your pics onto.... It is very easy to use, you just upload them there - open them, copy the URL, then when you are posting, click the "image" button and paste the URL pic there.