Hi Andy,
Thanks for reaching out.
You can do that directly to that shortcode, example
[block_grid type="two-up"] [block_grid_item] [image src="https://yourwebsite.com" alt="alt text" type="thumbnail"]<p>Image 1 Caption</p>[/block_grid_item] [block_grid_item] [image src="https://yourwebsite.com" alt="alt text" type="thumbnail"]<p>Image 1 Caption</p>[/block_grid_item] [/block_grid]
But if you’re looking for Wordpress builtin image caption, then it could be like this
[block_grid type="two-up"] [block_grid_item] [caption][image src="https://yourwebsite.com" alt="alt text" type="thumbnail"]<p>Image 1 Caption</p>[/caption][/block_grid_item] [block_grid_item] [caption][image src="https://yourwebsite.com" alt="alt text" type="thumbnail"]<p>Image 1 Caption</p>[/caption][/block_grid_item] [/block_grid]
Please check this for more information https://codex.wordpress.org/Caption_Shortcode
Thanks!