hi there, thank you! okay so :

1. press the '+' symbol in the top right of your carrd page ( while you're editing the site )
2. select 'gallery' from the drop-down menu
3. you can now add images to it. if you want to make it so the thumbnails can be clicked ( to view the full image ), go to 'mode' and change it from 'default' to 'lightbox'.
4. you can change how big the images are, how spaced out they are, if you want the thumbnails to play any animation when hovered, etc. from the tabs in the left corner of the page ( when the gallery element is clicked ). for my gallery, i have the 'images: on hover' set to 'grow'.
5. click done, then click the save button in the top right of your page and publish your site!

let me know if you have any other questions