Looking to extract url from an html tag

Hi guys...I've got a problem here, I'm trying this new eBay listing program and it presets how it places the images...it uses [PICTUREx] type tags to link to the attribute of where the pictures are stored on their server.

My problem is that I need to be able to resize them with just the basic width id in the html IMG tag to scale them. The problem is, there is no set way to extract the url to the pictures from [PICTUREx].

So I figured the best way was to use client side javascript to extract the url so then I can reinsert it into an html IMG tag so I can set the correct parameters. Unfortunately, every script I find has to be in the HEAD of the the document. And that's just not possible with an eBay listing.

This would work the same way that the simple ASP command TrimHTML() works, but from what I can tell, there is no simple way to do that with Javascript.

The [PICTUREx] tag (whatever that may be) would be converted into an HTML <img> tag before being sent to the browser anyway.

yes...but it would be converted into an entire <img src="http://img" width="too big" height="too big"> that's what I need to change.