第一张图片没有显示

I changed the html code below

<a href="<?php echo pietergoosen_get_first_image() ?>" title="<?php printf( the_title_attribute( 'echo=0' ) ); ?>" rel="lightbox">
          <?php echo pietergoosen_adjust_first_image(pietergoosen_get_first_image(), get_the_title(), 525, 0); ?>
       </a>

to the function below as I need more than one image size with an if statement

function pietergoosen_the_excerpt_image_output() {
$firstImage = pietergoosen_get_first_image();
$imageAttribute = esc_attr(the_title_attribute( 'echo=0' ));
$title = get_the_title(); 
if (isset($classes) && $classes == 'custom-sidebar-per-page-width'){
    $imageSize = '525';
} else {
    $imageSize = '380';
};

printf(
    '<a href="%1$s" title="%2$s" rel="lightbox"><pietergoosen_adjust_first_image(%1$s, %3$s, %4$d, %4$d)></a>',
        $firstImage,
        $imageAttribute,
        $title,
        $imageSize
);
 }

I call the function like this

<?php echo pietergoosen_the_excerpt_image_output(); ?>

but the image now does not show up anymore. Any advice on how to fix it, please

Your printf() call is a total mess. It's got syntax errors and is utterly unecessary:

echo "<a href='{$firstImage}' title='$imageAttribute' etc....";

is all that's necessary.

With your version:

printf('<a href="%1$s" title="%2$s" rel="lightbox"><p
       ^--single quoted string
                 ^^--- not a valid formatting char for printf
                   ^^--- variables are NOT interpolated in '-quoted strings

A proper (and still unnecessary) printf() would've been

printf('a href="%s" title="%s" ...., $var1, $var2);
                ^^--insert a string

The ONLY time a printf/sprintf() call would be useful in PHP is if you need to do special formatting on an inserted character, e.g.

printf('%05d', 12); // outputs 00012

Your new function is not returning a value to echo, it is printing from within the function. The last line should be:

return sprintf(
    '<a href="%1$s" title="%2$s" rel="lightbox"><pietergoosen_adjust_first_image(%1$s, %3$s, %4$d, %4$d)></a>',
        $firstImage,
        $imageAttribute,
        $title,
        $imageSize
);