Be Sure to Check If WordPress Has Support For Post Thumbnails When Building Themes

Post thumbnails were introduced in WordPress 2.9, but your users may be running an older version of WordPress. If you’re using post thumbnails in your theme, make sure to check if WordPress has support for them by adding something like this to your theme:

{code type=php}
if (function_exists(‘add_theme_support’)) {
add_theme_support(‘post-thumbnails’);
set_post_thumbnail_size(588, 250, true);
add_image_size(‘loopThumb’, 588, 125, true);
}
{/code}

The next step is to check if there is a post thumbnail assigned to the post when adding the tags to your template. If not, you will want to display an image that is set via the custom fields. Credit for this tip and more information on adding post thumbnails to your themes can be found at: onextrapixel.com.