How to extract postimage url from wordpress post?

Hey guys. So my question is just like in the title. How to extract postimage url from wordpress post? So for example if wordpress postimage() function outputs something like:

<img src="some_img_url.jpg">

I'm only interested in some_img_url.jpg.

The question sounds quite vague like this (to me at least), but at first glance – and if it's a quick client side solution that you're looking for(?) – I'd try using jQuery, more specifically the .attr() method. See the last example of the official documentation, for example: http://api.jquery.com/attr/