Hi,
You can also simplify your code a bit..
<?php $img=get_sub_field('testimonial_picture_or_logo'); ?>
<?php if( $img ): ?>
<div class="imgContainer2">
<img src="<?php echo $img['sizes']['medium']; ?>" width="<?php echo $img['sizes']['medium-width']; ?>" height="<?php echo $img['sizes']['medium-height']; ?>" alt="<?php echo $img['alt']; ?>" />
</div>
<?php endif; ?>