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.

Ricardo Moraleida

Default Values for Attachments gives you the ability to include default titles, captions or descriptions for every image uploaded using the Media Uploader.
You can still set individual values for field of every image. And you can unset defaults anytime.

Default Values For Attachments

Set default Title, Caption or Description for every image you upload.

Image Caption for WP vs Default Values For Attachments