skip_post_thumb not working / min image size?

For some reason skip_post_thumb is not working for skipping posts.

I have it set to 1 but the posts get images anyway.

I’m building a news agg. site and have feeds coming into my wordpress and some of them have little digg icons or a tracking pixels within the feed. Would rather avoid having those set as the thumbnails.

Could there be a way to specify a minimum image size for an image to become a thumbnail?

https://www.ads-software.com/extend/plugins/auto-post-thumbnail/