Try this in woocommerce/templates/single-product/product-thumbnails.php
Change this:
'<a href="%s" class="%s" title="%s" data-rel="prettyPhoto[product-gallery]">%s</p></a> ',
to this:
'<a href="%s" class="%s" title="%s" data-rel="prettyPhoto[product-gallery]">%s <p class="gallerytitle">%s</p></a> ',
And change this:
wp_get_attachment_image( $attachment_id, apply_filters( 'single_product_small_thumbnail_size', 'shop_thumbnail' ), 0, $props )
to this:
wp_get_attachment_image( $attachment_id, apply_filters( 'single_product_small_thumbnail_size', 'shop_thumbnail' ), 0, $props ),
esc_attr( $props['title'] )
and add some css in your css-file as example:
.gallerytitle {color: red !important; text-align: center !important; font-size: 11px !important;}
