Manual Attachment Links don't work?

[kau]

I know on this one forum I have, the guests can't see attachments

but even logged in as administrator, if I manually put in an image

like using HTML code:

<img src="http://www.sitename.com/directoryname/attachment.php?attachmentid=3&amp;stc=1&amp;d=1240688989">

If I manually put that hyperlink into the browser, an image appears

but I want that image on my forum display and it won't appear with the same hyperlink in the img src ?????

Any help would be appreciated explaining why this happens (keep in mind I have been logged in as admin the entire time) also if you view the pic manually it gets iin your cache so when you refresh the page its viewable the only way to test is to clear the cache of the image first to see what i am talking about