Problem:
The user would like to allow visitors to submit content and be able to add alt text and captions to images.
Solution:
When you "Allow Media Insert button in Post Content Rich Text Editor" when you create your Post Form, there will be a native WordPress Image upload screen where you can set those details just as used to in WordPress.
If you use a Types Image Custom Field, the uploader does not present those features and they would either need to be updated to the Attachment via a Custom PHP Script, using values coming from as example some "Single Line" Custom Types Fields, or, you would use the autogenerated values.
Both ways to upload images create a Post of type "attachment".
The different Meta-Data is stored as this:
Alt Text: uses the Post Meta Key _wp_attachment_image_alt
Caption: it's the actual Post Excerpt (Table column name post_excerpt)
Description: it's the actual Post Body (Table column name post_content)
With this, you could create some Single Line Fields where the users enter Alt, Description and Caption, then you grab this info with the get_post_meta function and update_post_meta or wp_update_post them to the Database via a CRED Save Data Function.
https://developer.wordpress.org/reference/functions/get_post_meta/
https://codex.wordpress.org/Function_Reference/update_post_meta
https://codex.wordpress.org/Function_Reference/wp_update_post
The CRED API will help you to fire those above functions when submitting the Post with CRED:
https://toolset.com/documentation/user-guides/cred-api/#csd
