To be honest, this is not a small job and really is going to take a lot of work to do this. But I’m thinking the best way is to edit the default styling of the images for the blog, recent posts, and or categories by adopting the styling from the Celestial theme and move them to the new theme, but in your own custom CSS and target the post type containers.
Focus on the classes “post-thumbnail” and “attachment-post-thumbnail” and “wp-post-image” and “aligncenter” “alignleft” and alignright.
The problem is that there are many kinds of images that can be used or used within a website and there are several locations of the CSS that manages styling for images.
Don’t forget to make backups before venturing into this project, because as I said, it’s not going to be too easy.