Milan Dinić

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Image descriptions are stored as content (field <code>post_content</code>) while captions are stored as excerpt (field <code>post_excerpt</code>). This means that descriptions are shown on single attachment pages, while captions are shown in <a href="https://codex.wordpress.org/Gallery_Shortcode" rel="nofollow ugc">galleries</a> and if image is embedded in a post content with a <code>[caption]</code> shortcode.
If you use images just in galleries, in order to have text below image in both places, you need to fill both fields.
This plugin changes that. You only fill description field and that text is used as caption of image in galleries.
There are no options or settings, just enable plugin and everything works automatically.

Descriptions as Captions in Galleries

Show image descriptions as captions in galleries.

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.

Descriptions as Captions in Galleries vs Default Values For Attachments