Re-size Single Image on Post

By default Twenty Twelve displays the featured image at the top of the post. It looks like that is what you are seeing.

The theme is designed to work with a featured image that is 625px wide.

A solution is to stop Twenty Twelve displaying the featured image when it displays the post – you can then insert the image you want at the size you want at the top of the post.

To do that you’ll need to create a child theme and edit the code.

To create a child theme: http://codex.wordpress.org/Child_Themes

To remove the featured image from posts see: http://wordpress.org/support/topic/disable-featured-image-as-header-in-twenty-twelve

You’ll need to put a copy of content.php in your child theme so you can edit it.