<img src="<?php print($base_path . drupal_get_path('theme', 'bartik')); ?>images/some-image.png">
The above code works only if you're in the homepage. If you go to any inner page, it looks for image inside the current URL, not just base URL.
So, it prints the image if you're under
www.address.com but if you're under
www.address.com/page, it doesn't print the image, because now it's looking for it inside e.g.
www.address.com/page/sites/all/themes/bartik/images/some-image.png the problem is, it includes the
In source, the image URL doesn't include the full path (which is the problem I believe?), only the