The div above has a background image that fills the div no matter how it is responsively sized or stretched.
You will use this most of the time!
Photos will scale up to the max cropped size
<img src="..." class="img-fluid" ...
Thumbnails will also scale up to max cropped size, and they get a white border and light outline.
Photo must be cropped square (same height and width), and you add a custom ".rounded-div" class which rounds the img corners.
Copy vector outline from Illustrator and paste into a div, then change the svg's "width" and "height" properties to 100%.