Hey Vonnagy,
have you tried adding captions to the uploaded images when using the “WP Media Gallery” as the source of images? You can also add captions post-facto by going to the “Media -> Library” admin menu, clicking on the needed image, and filling in the “caption” field – this is exactly how the theme demo front page does it. Please let me know if this still doesn’t work!
Sincerely,
Andrii / Satori Studio