How to add 10px between images using CSS?

If we use box-sizing: border-box; then you can just apply borders to specific elements without breaking the layout.

.sr-gallery li {
    margin: 0;
    padding: 0;
    float: left;
    width: 33.33%;
    overflow: hidden;
    text-align: center;
    border-bottom: 10px solid white;
    box-sizing: border-box;
}

.gallery-col2 li {
    width: 50%;
    border-right: 5px solid white;
}
.gallery-col2 li + li {
    border-left: 5px solid white;
    border-right: none;
}