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.