Attachment Images

I just changed my theme, and when I set a new ‘featured image’ to a new post, some of the images in my media library will show up on the page as a thumbnail and others will appear full size. However this wasn’t the case with my old theme: all images were thumbnails, as I want, mainly for sharing on social media. Some of my photos in my media library are listed as ‘attached’ and some aren’t. Why can’t I get all the images to show as thumbnails, and why are some displaying themselves big? My URL: massintelagency.com

Thanks,

Chris