Hi Ryan,
If you look at where the border-radius ends up (on the a, instead of the img) you will understand why the overflow: hidden is necessary (for which you don’t need a single line of custom CSS, by the way).
Nevertheless, we have changed the behavior in Bricks 1.9.7 to add the border-radius directly to the img instead of the a. However, this obviously leads to problems with figure tags, so it is currently uncertain whether this change will last: WAIT: IMG and FIGURE - new "border-radius: inherit" since 1.9.7?