This is one of the simple effect with simple css code. Simply highlight the image using border you can use this one line code on hover the image. Just call one class to the particular image and write the code for class hover as a border :3px solid #000; you can choose any other colors and border width as you want. this type of hover effect will highlight the image and this will useful for the gallery page as well.
Code for simple border effect :
<title>Simple border Effect on hover image</title>
border:3px solid #000;
<img src="border-effect-css.jpg" alt="border-effect-on-hover-image" style="width:450px;" class="border">