theres no simple way to do that – and that would require you creating a lot of CSS.
Instead of that you would need to add a CSS Class to the images YOU DO NOT want the shadow applied to:
So change the CSS to this:
.single-post .entry-content .wp-block-image:not(.no-shadow) img {
box-shadow: 0 0 8px 4px rgba(0,0,0,0.25);
}
Then any Image you do not want the shadow applied to, select the Image Block, in the Settings sidebar > Advanced –> Additional CSS Class(es) field add: no-shadow