Border highlight on hover

I want to create a highlighting effect on an image when I hover my mouse over it. I have a few draggable images that are layered over each other in a pile. When I drag an image that is on a lower layer underneath images in the above layers it highlights those too. Is there a way I can make the selected image being dragged remain highlighted until it is unclicked? And is there a way I can make the selected image ignore all the other unselected images so that they don't highlight?