Display image (template files)

Hi. I want to let my customer choose any image (in my backend) and then it will be displayed in a template file (header.php, footer.php, loop.php, etc). i’ve found Image Widget, but it just show images in the sidebar. I want something like in my template file:
<?php wp_plugin_that_shows_a_simple_image(); ?>
Which plugin could I use? I’ve searched a lot. WP 3. Thank you.