Are you making your images featured images? Most themes grab the featured image, not just the ‘first’ image in the post.
You need your featured images added like this: Image preview under post headings in Solo - #2 by Cathy_Sarisky
Now, having said that, it sounds like maybe your port didn’t work as well as it could have. Ideally, whatever tool you used to do the porting would have set up your featured images for you, assuming you had them set up in WordPress.