{ z-index: x; }

Interactive example

Position all images:

z-index img#red:

z-index img#green:

z-index img#blue:

/* DOM: body > main > div.container > (img#red ^ img#green ^ img#blue) */
div.container {
    position: relative;
}
img {
    margin-bottom: 24px;
}
img#red {
    
}
img#green {
    
}
img#blue {
    
}