Lightboxes fetch text from the opener link's title attribute and the thumbnail image's alt attribute. These are also used for SEO. You can directly influence what text JIG adds to these fields. Go to JIG settings > What text to show inside the lightbox and choose options according to your needs. The caption's placement varies from lightbox to lightbox, so you might want to check on these settings after choosing a different lightbox.
- If you don't wish to use captions at all, because you don't set any text on the pictures then you can turn these settings off. It prevents the filename-derived title from showing in the lightbox.
- To actually change the text that shows, edit the picture at your source, such as in the WP Media Library.
- In order to show different lightbox captions for mobile users, create a custom preset according to your needs and select it in the Shortcode Editor > General tab > Mobile preset setting.