image borders in Hatch theme

My site is http://www.danielmcgowanmusic.com

I’d like to add a black border around the 12 images. When I did this by setting a border in the style.css file (via the WP editor) the borders only showed up on the left and top. I realize this is because the borders increase the size of the image, causing the bottom and right to be cropped.

Is there a way to then correct the size of the box in which the entire image (picture plus border) sits? Or perhaps to add the border to the box holding the image, rather than the image itself? (Sorry if my nomenclature is crude.)

FYI, as these are featured images (rather than post content) I don’t seem to able to access the image’s advanced settings, which easily allows you to add borders.

Thank you!