First up, download the Template Conditionals plugin.
Install it, as you would any other MyBB plugin.
Now, head to Templates > Your Theme > Member Templates > member_profile
After <body> add the following code, changing the URL to the image you want to use and changing YOUR ID to your ID:
now in global.css
PHP Code:
.vanity {
position: fixed;
bottom: 0px;
right: 0px;
}