Documentation on the_terms function-
<?php the_terms( $id, $taxonomy, $before, $sep, $after ); ?>
Looks like your providing the image as the before parameter, so i'ts only being printed once. Try putting the image in twice, so it gets printed for both the before and the seperation parameters.
<?php the_terms( $post->ID, 'case_tags', '<img src="theimage" />', '<img src="theimage" />','<br /> '); ?></div><!--end .case-tags -->