<script src="JavaScript/jquery-3.2.1.min.js" type="text/javascript"></script>

<script type="text/javascript">
    function scal(p=0) {
        $(".imm").css("height", $(".imm").height()+p);
    }
</script>

<style type="text/css">
    .gr {
        width: 300px;
        height: 300px;
        overflow: scroll;
    }

    .imm {
        height: 300px;
    }
</style>

<div class="gr">
    <img class='imm' src="Files/Images/1505855465.jpg">
</div>
<button onclick="scal(50); return false;">+</button>
<button onclick="scal(-50); return false;">-</button>