It's so simple that I am surprised by your request, anyway ;)
- In Image addon > Style (Tab) > CSS Class field add
img-shadow
- Then use that Custom CSS
.img-shadow img {box-shadow: 8px 8px 15px rgba(0, 0, 0, 0.5);}
and for all Image addons it would be
.sppb-addon-single-image-container img {box-shadow: 8px 8px 15px rgba(0, 0, 0, 0.5);}
but then must be used inside Page Settings > Custom CSS (for all images on that page), or inside Template Options > Custom CSS (for all images on whole website).
Here my CSS help ends. You're welcome.