If you just want to refer to the original post, and only the original post, you can right click on the number at the top right of the post (for example, the #3 on this post) and select copy link location, copy shortcut, or whatever it is for the browser you are using. Then paste it in a new post. Then when someone clicks on it, it'll automatically take them to that individual post. Here's Jim's post above as an example:
http://www.sawmillcreek.org/showpost.php?p=209214&postcount=2
If you want to link to a whole thread, and have it go directly to a post, then one way to do it is to copy and paste the link to the thread, then add #post followed by the post number that you want it to go to. You can get the post number by moving the mouse over the number to the top right and looking in the bottom of the browser at the status bar. The p=xxxxxx is the post number. Surely there's an easier way to do it, but this way works. Again, Jim's post as an example:
http://www.sawmillcreek.org/showthread.php?t=23173#post209214
If you want to basically copy an attached image in to a new post, probably the easiest way is to just copy the html from the old post. For example, I'll use this thread:
http://www.sawmillcreek.org/showthread.php?t=23175
Right click on the image and select properties. In the image address you can get the attachment id, for the first image it's 22572. Go to view, page source, and it'll open the page source in a text editor. Do a search for the attachment id number, and it'll go to the source for that image. If you want it to be exactly like normal attached images (with the border around it, and text at bottom) you can copy and paste from the fieldset class part down to the /fieldset. Like I did here:
<fieldset class="fieldset">
<legend>Attached Images</legend>
<table cellpadding="0" cellspacing="3" border="0">
<img src="attachment.php?attachmentid=22572" target="_blank"><br />
<span class="smallfont">(08-13-05 005s.jpg, 89.8 KB bytes)</span><br /><br />
</table>
</fieldset>
Or another way is to just get the image attachment address and paste it in an img src tag then just close it. It'll post it just like a regular image. Again, there almost has to be an easier way to do it, but either of these ways will work. I'm not sure why you can't just use the attachment address with the bracketed IMG tags that the message editor uses, but it'll work fine with the img src= way of doing it.