adding image in the header of my dev widget

From your post what i understood is u need to show images saved in your plugin folder.if yes then you can go with plugin_url to link images in a plugin directoy without specifying the plugin folder name.
Example:

<?php
echo '<img src="' .  plugins_url('/images/header-background.png', __FILE__) . '" > ';
 ?>

The ouput will be like

<img src="http://www.example.com/wp-content/plugins/my-plugin/images/header-background.png">.

If you are using the plugins_url() function in a file that is nested inside a subdirectory of your plugin directory, you should use like this:

<?php
echo '<img src="' . plugins_url( 'images/header-background.png' , dirname(__FILE__) ) . '" > ';
?>

If you want to know more about plugin_url() refer here