Go to Main » Customize design » Editing templates » File Catalog » Page with Entry full text and its comments and then find
Code
<a href="$FILE_URL$"><!--<s5223>-->Download from this server<!--</s>--></a>
and substitute it with
Code
<a href="$FILE_URL$"><img src="IMAGE.PNG" alt="alt text" /></a>
[p.s.] Your Picture
Video Tutorial: