[Keeper]

<img id="imageID" src="image_1.png"/>
<input id="numImage"  type="text" name="test" class="test" value="1"/>
<button onclick="chngImage();">Сменить</button>
<script type="text/javascript">
function chngImage() {
var num = getElementById('numImage').value,
     img = new Image();
     img.src = 'image_'+num+'.png';
    getElementById('imageID').src = img.src;
}
</script>