<?php
$img=get_sub_field('testimonial_picture_or_logo');
$alt= $img['alt'];
$size = 'medium'; // (thumbnail, medium, large, full or custom size)
$thumb = $img['sizes'][ $size ];
if ($img): ?>
<div class="imgContainer2">
<div>
<img src="<?php echo $thumb; ?>" alt="<?php echo $alt ?>">
</div>
</div>
<?php endif;
?>
This one is proper way to get img with sizes defined in database……..
this code is part of ACF Pro, where i am using this img from sub filed, that can be changed to get_field