How can I find out what the_post_thumbnail outputs?

I’m trying to learn about WP, and this function in particular. I read the function reference and I see it takes two args, and that another function should be used.

What I’m trying to do is relate this function’s output to what I get in the page source. So, I want to know what it does and how it does it. I want to change the output so that this particular “responsive” theme is more responsive. I don’t want to change the function. I just want to know the limits I have to deal with. I will probably use the get_the_post_thumbnail function, so, I guess I need to get married to that one too.

So, if someone could point me in the right place, I would appreciate it.
Thanks