If the widget route is the direction you want to go, first you need to register a new widget in your themes function.php:
if ( function_exists('register_sidebar') )
'name' => 'Post Image Widget',
'before_widget' => '',
'after_widget' => '',
'before_title' => '',
'after_title' => '',
Then you simply need to add the widget to your themes post.php before The Loop:
<?php if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar('Post Image Widget') ) : ?>
Post Image Widget being whatever you want to name the widget. You can then add a text widget with with your <img /> tag in the body. If you're looking for a more user friendly route, add the above code to your theme and install the Image Widget.
There's a good "widgetizing" tutorial here.