Defaut image value not showing up
If the default image was set correctly then the newly saved article nodes should pick it up. Check if the website's log show any related error messages.

If you want to go with another method then just:


Copy node.tpl.php in your website's themes folder to node--article.tpl.php;
Open node--article.tpl.php in editor and place the following code just before print render($content);:


&lt;img src="/path-to/image/image_file_name.ext"&gt;


Save.
Clear your website's cache and revisit any article node.