I know Python is good with JSON and I found it not-too-difficult to get a feel for while I was playing with Sublime Text plugins. I'm not a software guy, really, but it seemed like with aggressive Googling and a little perseverance it was easy enough to figure out. As for your regex question, if it's always going to be a png file, this clumsily but accurately matches just the string you need, according to Patterns:
(?<=")http.+png(?=",)
But, again, not a software guy, so there's probably a more elegant way.