Using extra field for uploading image

You probably do know the answer. A file upload extra field is designed to be able to upload a downloadable file, therefore the result in the template to get this field will be a clickable download link.

My suggestion to you would be to use a drop down menu text field and input the ratings that you allow. (e.g. 1, 2, 3, 4, 5) Create corresponding images to display. Then use smarty in the template to check for the value and display the proper image either using a direct <img> or css using a defined class with a background image. (assuming you create a folder in the root directory called, "ratings" and has images 1.jpg ... 5.jpg)
example using css:
{get_extra_field id=$view fid=45 assign="rating"}
{if $rating}
<div class="ranking rank_{$rating}">&nbsp;</div>
{/if}

In stylesheet:
div.ranking {width:200px;height:30px;margin:5px;}
.rank_1 {background: url('ratings/1.jpg')}
...
.rank_5 {background: url('ratings/5.jpg')}

example for in-template smarty:
<!-- get the field value and assign it to a variable called "rating" -->
{get_extra_field id=$view fid=45 assign="rating"}

<!-- Check if there actually is a rating -->
{if $rating}
<div> <!-- start a div to display the image in -->
{if $rating == 1} <!-- use checks to display the proper image -->
<img src="ratings/1.jpg">
{elseif $rating == 2}
<img src="ratings/2.jpg">
...
{/if}
</div>
{/if}