How to make full-sized image show the first time you click on a post

What version of WordPress are you using?

And what theme?

Ok. There is a specific support forum for Twenty Twelve here which may be more help:

http://wordpress.org/support/theme/twentytwelve

But here are a few things to look for. First, the page that you’re seeing on your second click is the image’s “attachment page,” which is generated by the Media Library. So, what you want is a way to display the attachment page as a post, which I can’t figure out either (sorry!).

You could create a child theme to Twenty Twelve and define a new post type that mimics the attachment pages – that’s probably the easiest thing. But I’m sure others may have better ideas.