There are different ways to create custom bullet points, either with the list-style-image CSS property or with a background image or with generated content (CSS pseudo-elements :before/:after, and then a background on those).
The best practice would be an internal stylesheet in the document header (similar to what John Huebner suggested), i. e. in header.php, and then add a class to the list (or an ancestor) in question to apply this style to that element.
Another way would be inline-styles on the list items themselves, like:
<li style="background-image: url("<?php the_field('bullet_image'); ?>">item text</li>
But the first method would be preferable as it is more flexible in the end.
