Photo Outlines in a Gallery Grid

Hi - I have images in a photo gallery grid block with white background. Some of the images are white so I would like to add a 1 pt. gray outline for all images. Does anybody know of any code that would do this? Thanks in advance.

Try this CSS, if it doesn't work, please share link to page where you use gallery block

.slide {
	border: 1px solid black;
}