One of the countless things that I love about WordPress is how easy and efficient the process of adding new content is. All you have to do is provide the art and copy and your theme will take care of the rest. While this workflow is usually smooth, there are times the end user can miss a step or two. I’ve noticed when working on larger sites with multiple authors one of the most common things to slip through the cracks is setting a featured image. While this shouldn’t break your theme, it can create layout inconsistencies or (shudder) broken images. Lucky for us, we can easily provide a fallback, and I’ll tell you how.
WordPress Post Thumbnails
Post Thumbnails were introduced in WordPress 2.9 [Source: WordPress Codex] and have since become somewhat of a standard for theme developers. If a theme declares support for this functionality, it allows authors to set an image that will represent Posts, Pages, or Custom Post Types. How this image is utilized is completely dependent on the theme. A lot of themes will use a featured image to accompany excerpts, but that’s just one example.
Setting A Fallback
If your theme uses Post Thumbnails, it is probably wise to set a backup. That way if you (or one of your authors) forgets to set a featured image before publishing a post it will not break the layout. One way to do this is by grabbing the first image from the post, and returning it if no post thumbnail exists. First, add the following to your functions.php:
//Call the first uploaded image in the post
function fallback_image() {
$files = get_children('post_parent='.get_the_ID().'&post_type=attachment
&post_mime_type=image&order=desc');
if($files) :
$keys = array_reverse(array_keys($files));
$j=0;
$num = $keys[$j];
$image=wp_get_attachment_image($num, 'thumbnail', true); //Set your image size and crop to true or false here
$imagepieces = explode('"', $image);
$imagepath = $imagepieces[1];
$fallback=wp_get_attachment_url($num);
$template=get_template_directory();
$the_title=get_the_title();
print "<img class='featured' alt='$the_title' src='$fallback' />";
endif;
}
Now, determine where in your theme the featured image is used (Hint: Check index.php, loop.php, home.php, etc.). Look for the call to get_the_post_thumbnail and replace it with the following:
<?php if ( (function_exists('has_post_thumbnail')) && (has_post_thumbnail()) ) { //If a featured image was set, return it
echo get_the_post_thumbnail($post->ID);
} else { //If it wasn't set, return the fallback
echo fallback_image();
} ?>
Note: I am assuming that your theme has already established support for Post Thumbnails. If it hasn’t you’ll need to add that functionality. Don’t worry, it’s easy. Just look to the Codex to learn how to enable support for Post Thumbnails.
Conclusion
This is just one way you can handle fallbacks for featured images. I have seen a lot of themes let allow you set a single default image, a company logo for example, as a fallback and that seems to work perfectly fine as well. The decision is ultimately up to you.
https://www.searchenginepeople.com/blog/wordpress-thumbnail-fallback.html