The quick & dirty way, to set the title as caption, would be to use SQL (untested):
SET post_excerpt = post_title
post_excerpt = ''
AND post_type = 'attachment'
AND post_status = 'inherit'
AND post_mime_type = 'image/jpeg'
AND ID = 123
Here we target the jpeg image with ID
123 and empty caption.
Note I added the
ID = 123 and
post_mime_type = 'image/jpeg' as extra
restrictions that you can adjust while testing. Also remember to
adjust the table name of
WARNING: Backup your database before testing!
If you're looking for a dynamic way, then my answer here might be related.
It might also be possible for you to add the caption while uploading your images:
* Automatically set the title as caption, when uploading an attachment.
* @see https://wordpress.stackexchange.com/a/188708/26350
add_filter( 'wp_insert_attachment_data', function( $data, $postarr )
// Let's target only the uploading process and not the updating of attachments:
if( empty( $data['post_excerpt'] ) && isset( $postarr['ID'] ) && 0 == $postarr['ID'] )
$data['post_excerpt'] = $data['post_title'];
}, 10, 2 );