This is gallery and to replace those images with own, you need to change it's ID's but those id's you can find under Media section where all images are. More details about WP Gallery you can find on https://codex.wordpress.org/The_WordPress_Gallery
But it's not a button. It was made with Gallery Shortcode in a column. On page we gave you above, is a link to gallery shortcode https://codex.wordpress.org/Gallery_Shortcode as well.
Comments
which images you mean exactly?
All details about images sizes you can find on http://forum.muffingroup.com/betheme/discussion/938/problem-with-image-size-in-shop-slider-media-images-settings-sizes