It is actually much easier than you think to implement. First assign your image an ID
Next go to the custom code section of the page and add this css with as many ID’s as you wish to apply the drop shadow to 
<style>
#eraser, #pencil, #sketchbook {
-webkit-filter: drop-shadow(5px 5px 5px #222 );
filter: drop-shadow(5px 5px 5px #222);
}
</style>
Just a heads up, it will only show published. Will not show in preview mode.