The same way your added the custom image URL to the image tag above it
<img src="<?php echo $image['sizes']['galleryImage']; ?>" alt="">
<a href="<?php echo $image['sizes']['OtheCustomImageSize']; ?>" class="project-caption"><span>+</span></a>
When in doubt, you can always output the entire image array to see what’s in it
echo '<pre>'; print_r($image); echo '</pre>';