wpcocktail

The Site Title Caption Modifier is a simple WordPress plugin that sets the caption of all images in your WordPress site to the site title. This is particularly useful for maintaining a consistent branding or style across all posts and pages. The plugin works by hooking into the WordPress ‘img_caption_shortcode’ filter and modifying the caption output.

Image Caption for WP

Automatically sets the caption of all images to your site's title.

hornament

Plugin accesses the caption shortcode via <code>add_filter( \'img_caption_shortcode\', ... )</code>.
By default, image authors are read out of the description of the respective image. Therefore, they have to be tagged in a specific way: 
In the description of the respective image, accessible under ‘Media’ > ‘Edit’ > ‘Description’, the image author (or multiple image authors) have to be wrapped in <code><author></author></code> tags.
If there are no authors in image description, the plugin tries to read authors from the image’s Exif information. If successful, image authors are automatically saved into image description.

fla_Image Caption

Plugin to insert image authors automatically.

Image Caption for WP vs fla_Image Caption