1. Make a custom user field
2. Title Postbit Cover or Anything you want
3. Field ID can be anything i suggest no space on it
4. Description : JPG, PNG, GIF (Link of the Image)
5. Display Location : Personal Details
6. Field Type: Single Line text Box
7. General Option: User editable, Moderator Editable
8. go to templates >> search message_macros >> paste this after xf:macro
done 
you can customize the css though if you want to ask anything dont hesitate to comment
2. Title Postbit Cover or Anything you want
3. Field ID can be anything i suggest no space on it
4. Description : JPG, PNG, GIF (Link of the Image)
5. Display Location : Personal Details
6. Field Type: Single Line text Box
7. General Option: User editable, Moderator Editable
8. go to templates >> search message_macros >> paste this after xf:macro
Code:
<div class="postbit_background" style="background-image: url({$user.Profile.custom_fields.YourFieldID});"></div>
9. and add this at extra.css
Code:
.postbit_background {
position:absolute;
height:120px;
width:170px;
opacity:.40;
}