I have over 1500 posts I imported from Tumblr. While the images were imported into the gallery, I need to make sure there's a featured image for each. How do I make the images (12 per post minus 30 that don't have images) a featured image automatically?
You can run a script that will programmatically set the featured image for each post. You can use the first attached image for this. To get the attached images run a query for the posts and loop through each one and use get_children() setting the post_parent to the current post id in your loop.
You would need to put this code somewhere it can get executed like a custom page template that you would load only one time to set the featured images.
You could also use a plugin I wrote that will do this via an admin tools page via ajax. http://wordpress.org/extend/plugins/media-tools/