How to prevent WordPress from cropping the white border from my featured images

I’m creating an art portfolio site using the Koncept theme. My images are Gifs and all have solid colour backgrounds. This template doesn’t have any spacing between each element on the masonry grid, and I’m not happy with how they look all flat against each other.

I created a white border of about ten pixels around each image as a way of faking the appearance of spacing, but when I upload them and place them in the ‘Featured Image” area of the ‘Portfolio’ pages, WordPress crops away the white border as if it never existed.

I thought that unchecking the option “Crop thumbnail to exact dimensions (normally thumbnails are proportional)” in Dashboard>Settings>Media might have been the answer but it didn’t work.

why is this? Is it because the a white pixel has zero value? If I change the border colour to black, WordPress will keep the border. I thought it would be a simple fix but it isn’t, and it’s been pretty frustration.

Any help with this would be appreciated. Thank you!