How to make image/div smaller when LMB is being pressed?

This is very easy to do with CSS only, using the :active pseudo-selector (that corresponds to "mousedown"). Also why not use Flex for easy centering.

.container {
  border: #00f solid 1px;
  width: 250px;
  height: 250px;
  display: flex;
  justify-content: center;
  align-items: center;
}
.container img {
  transition: all 0.3s ease-in-out;
  transform: scale(1);
}
.container img:active {
  transform: scale(0.8);
}
<div class="container"><img src="https://via.placeholder.com/150"/></div>


Try

transform: scale(0.7)

instead of height. It will be centered and it's more efficient (GPU usage)


place the LMB on the image not the div and use width not height

$(document).ready(function() {
    $("#pic").mousedown(function() {
    
    
        $("#pic").css("width","70%");
    
    });
    
    $("#pic").mouseup(function() {
   
    
        $("#pic").css("width","100%");
    
    });
});
.centered {
    
    text-align: center;
    display: block;
    
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<body>
    
        <div id = "banana" class = "centered">
            <img id='pic' src="https://picsum.photos/300" alt="banana.png">
        </div>
    
</body>