Karen,
If I didn’t have to worry about the thousands of sites using the theme already, I would so change how the slideshow works here. But I can’t change it without disrupting them unfortunately.
Back when I made this, I didn’t have as many options to let WordPress know which images to include in the slideshow. The only way to make that connection is to set it so that all images that are uploaded while editing that page/post will be the ones used in the slideshow. You don’t have to insert the images into the page/post content, but you do have to upload them on that page.
So if you need to have a separate thumbnail gallery on that page, upload those images by going to Media in your dashboard and selecting Add New. You can upload the images there. Then go back to the page and insert a gallery. You’ll then be able to pick the images out of your library to include in the thumbnail gallery.
The size of the thumbnails is set in WordPress’ media settings. To change those, go to Settings in your dashboard and click on Media. You’ll be able to adjust the size and crop there.
Truthfully if I was setting up a site with Foundation at this time, I would probably install a slider plugin like Soliloquy (https://wordpress.org/plugins/soliloquy-lite/) and insert it into the page where I wanted the slider. That would give more options and flexibility.
Let me know if I can help,
Bill