Thank you so much for using our theme Infinite Photography. No Image Available will appear only on post not in header image. Please add the featured image on the post where the No Image Available is appeared. Please have a look here to know about how to add featured image on a post. https://www.acmethemes.com/blog/2016/04/set-featured-image-on-post/
Hope this will solve your problem. If the problem is still not solved, please let us know. We will provide the alternative solution too.