Okay, so I was having the same sort of problem I was having previously–namely, even when you say you want attachment pages to show the full image, they still don’t. You have to click through to the file itself in order to see the full thing. I mentioned that with the pre-2.5 fix here.

Similar fix, but the code’s changed a little. Again, look for *whateveryoursiteis*/wp-includes/post-template.php file.

Look for this code. For me, it starts at line 504.

function prepend_attachment($content) {
	global $post;

	if ( empty($post->post_type) || $post->post_type != 'attachment' )
		return $content;

	$p = '

'; // show the medium sized image representation of the attachment if available, and link to the raw file $p .= wp_get_attachment_link(0, 'medium', false); $p .= '

'; $p = apply_filters('prepend_attachment', $p); return "$p\n$content"; }

You should change the medium there to full.

Now, if you’re like me and paranoid about coding you might ask yourself: before I was giving a maximum size…what happens now? Well, what happens now (I would assume) is that if you load an image that’s too big you’ll blow up your theme. They’ve obviously taken the shrinking bit out of the code and put it elsewhere. Where, I don’t know. So feel free to tinker if you’re the tinkering sort, but your safest bet is just to ensure your image width doesn’t go beyond what your theme’s content area would allow.

Let me know if you find any more/better ways to screw around with this.

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