Put file download image
You can modify your theme's CSS to give every File Field a specific background image.  You would add this to your theme's stylesheet, looking for a "style.css" file in your theme folder.  The generic code for the background image would be:

.node-your-node-type .field-type-file {
  background: url("/sites/all/themes/yourtheme/yourbackgroundimage.png") no-repeat scroll 0 0 transparent;
}