What are the meta fields for an attachment?

You can easily inspect all the metadata for any post like so:

// 123 is the (attachment) post ID
var_dump( get_post_meta( 123 ) );

And in a default WordPress setup with no plugins or theme which add custom metadata to the image attachment post, the metadata you’d get are:

• _wp_attached_file — string, the image file (path and name) for the attachment post

• _wp_attachment_metadata — array (serialized), the sizes for the image like ‘thumbnail’ (including original size), and metadata from EXIF/IPTC info.

• _wp_attachment_image_alt — string, the image’s alternative text

As for the other details like the image title, they are in the post data (in the posts database table), so you can for example use wp_update_post() to update these details:

• post_title — image title

• post_excerpt — image caption

• post_content — image description