Problem with is_tax in WP_Query loop

The codex on is_tax:

This Conditional Tag checks if a custom taxonomy archive page is being displayed.

That is not the condition you want to check for.

Take a look at the has_term function and use it like so:

if( has_term( 'tennis-ball', 'ball-types', $post->ID ) ) { 
    /* SHOW THE TENNIS BALL GRAPHIC */
} else {
    /* SHOW THE PLACEHOLDER GRAPHIC */
}

And as an aside, remove some of the <?php opening and closing tags. They serve no purpose and simply make your code less legible.