My feature images are showing up twice on the preview page

I think you given like echo the_post_thumbnail($post_id); No need of giving echo if we are using the_post_thumbnail().It by default echoing the result.So here, it will display the image twice. If we use get_the_post_thumbnail(), then echo is needed. So use either echo get_the_post_thumbnail(); or the_post_thumbnail(); Try this..

Images are not shown

It seems that there is problem with your image thumbnail. May be it doesn’t exists. You can fix this by instaling Regenerate thumbnails plugin. Then generate thumbnails for all media files. Another way is to include image in post or page with full url. You can do this by selecting image size to “Full Size” … Read more