Bushwick only displays featured images in the custom header area, so if you define a featured image for a post the theme will display the post’s featured image as the header graphic on the left side of the page, as long as the image is at least 900 px by 1600 px. Bushwick uses a custom function in custom-header.php to achieve this, and isn’t something that can be easily changed to the standard get_the_post_thumbnail(); function mentioned in that plugin’s documentation.
If you absolutely need to use featured images that aren’t in your Media Library, you may want to look at a different theme.
Part of that reason is because Buschwick handles it a little different. Pretty neat, my opinion, how it handles the header/featured image.
What it does is run a quick check to see if it is a single post and if it is it will use the post-thumbnail if there is one. If there isn’t one it won’t use it as the background image.
One way I can think of would be creating a child theme, and passing it some information. It all really depends on how comfortable you are with code. 🙂
I tried this:
add_filter( 'bushwick_custom_header_args', 'child_args' );
$args = array(
'wp_head_callback' => 'child_callback'
// What will be used instead
The downside is, of course, that we don’t really have access to your plugins code so I can’t really say what to use inside the callback function. 🙂
Hope that helps you out at least some. If not, don’t hesitate to ask questions. 🙂
The bug that was causing the custom header to not display on single posts has been fixed. We’ve resubmitted the theme to the WordPress.org repository, but if you’d like to download it now, you can get it from WordPress.com: