Okay, I can't give a perfect answer for what you are asking for but here is what I did. I was at first having issues with the image size in the post. I could not change it with the Mod, and was a little frustrated. Also when I would select the picture it would open in a new tab, which I didn't like, but it was okay,
I decided to use Firebug and check errors on the page. I noticed that Highslide was missing a file that was being requested. I did a quick google search and found the Index of highslide. What is nice about the index is that you may download only the file that you need to include in the highslide folder on the forum. The one that I was missing was called highslide.min.js I found it on the site I'm about to give you and included it in the highslide folder and now I can size the image on the page using the Mod, and it blows up on the same page.
There are two differences with the way that mine is working right now than the one that you originally reworked. When you hover the mouse over the image it shows a "+" sign inside a magnifying glass. When you select it the image shows full size and the mouse turns into a "-" sign in a magnifying glass. I have not checked out anything else on the use of it with signatures, and it seems to work with the CMS, but my forum is new, so I only have one article on it at this time.
Index of Highslide
Firebug for Firefox, there is a lite version for other browsers
Note: If you are not familiar with Firebug, then watch the video they have on the link I provided above. It should answer almost all of your questions.
Picture provided below. I used a full size image which you will see in the second pic. Also notice that it is still on my forum, and not in a new tab. So far the fix that billstelling made and me adding the missing folder is working nicely. I just don't have some of the button features, but I don't think that it's necessary. You never leave the page you are on and it's just a simple increase in size and decrease in size.
Edit: Forgot to mention that I'm running 4.1.11 PL1. I'm going to update soon, so I will post back if the mod is still working or not.