Well I disagree, I think it would be very easy to get around this.
The easiest way would be to mod the code that inserts the URL on the text box when you upload to insert the full URL, all that takes it's inserting "http://" & Request.ServerVariables("HTTP_HOST") somewhere on the js function that inserts the link and changing the path to a virtual path on the admin section. There's also other ways to do it like using the var that holds the URL for the forum, etc.
Or if you prefer on post_message.asp look for something like this:
'Initailise the e-mail body variable with the body of the e-mail
strEmailMessage = strTxtHi & " " & decodeString(strUserName) & ","
strEmailMessage = strEmailMessage & "<br /><br />" & strTxtEmailAMeesageHasBeenPosted & " " & strMainForumName & " " & strTxtThatYouAskedKeepAnEyeOn
strEmailMessage = strEmailMessage & "<br /><br />" & strTxtEmailClickOnLinkBelowToView & " : -"
strEmailMessage = strEmailMessage & "<br /><a href=""" & strForumPath & "/forum_posts.asp?TID=" & lngTopicID & "&TPN=" & intReturnPageNum & """>" & strForumPath & "/forum_posts.asp?TID=" & lngTopicID & "&TPN=" & intReturnPageNum & "</a>"
strEmailMessage = strEmailMessage & "<br /><br />" & strTxtClickTheLinkBelowToUnsubscribe & " : -"
strEmailMessage = strEmailMessage & "<br /><a href=""" & strForumPath & "/email_notify.asp?TID=" & lngTopicID & "&FID=" & intForumID & "&M=Unsubscribe"">" & strForumPath & "/email_notify.asp?TID=" & lngTopicID & "&FID=" & intForumID & "&M=Unsubscribe</a>"
and add something like this
strEmailMessage = Replace(strEmailMessage, "yourimagepath", "yourfullurl", 1, -1, 1)
That should do it, you'll have to replace yourimagepath with the path to the uploads folder exactly how you entered it on the admin section, and yourfullurl with the full url to that path.
That's just an example, it would not always work correctly, like if somebody post the full url it will get messed up and if by coincidence somebody post another link which contains the text string on yourimagepath it will also be converted. That could be fixed (or chances of it happening can be minimized greatly) by checking that the link is inside img tags and that it doesn't begin with http or www.