Justin Tadlock

<p>The Custom Classes plugin allows you to create custom classes for posts (when <code>post_class()</code> is called) and for the <code><body></code> class (when <code>body_class()</code> is called).  It creates a meta box on the edit post and edit term screens in the admin with input boxes for adding your custom classes.</p>
<p>Custom post classes are added whenever your theme calls the <code>post_class()</code> function, which is generally whenever a post is shown.  Custom body classes are added on the single view of the post whenever your theme calls the <code>body_class()</code> function.</p>
<h3>Like this plugin?</h3>
<p>Please consider helping the cause by:</p>
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<li><a href="https://themehybrid.com/donate" rel="nofollow ugc">Making a donation</a>.</li>
<li><a href="https://themehybrid.com/club" rel="nofollow ugc">Signing up at my site</a>.</li>
<li><a href="https://wordpress.org/support/view/plugin-reviews/custom-classes?rate=5#postform" rel="ugc">Rating the plugin</a>.</li>
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<h3>Professional Support</h3>
<p>If you need professional plugin support from me, the plugin author, you can access the support forums at <a href="https://themehybrid.com/board/topics" rel="nofollow ugc">Theme Hybrid</a>, which is a professional WordPress help/support site where I handle support for all my plugins and themes for a community of 75,000+ users (and growing).</p>
<h3>Plugin Development</h3>
<p>If you’re a theme author, plugin author, or just a code hobbyist, you can follow the development of this plugin on it’s <a href="https://github.com/justintadlock/custom-classes" rel="nofollow ugc">GitHub repository</a>.</p>
<h3>Will this work with custom post types?</h3>
<p>Certainly.  This plugin works with any public post type on your site.  It’s not just limited to the “post” post type.</p>
<h3>It’s not working!</h3>
<p>Most likely, this means your theme isn’t using the appropriate functions (<code>post_class()</code> and <code>body_class()</code>).  You’ll need to talk to your theme author to get them to fix this.  Or, better yet, use a correctly-coded theme from <a href="https://themehybrid.com/themes" rel="nofollow ugc">Theme Hybrid</a>!</p>


Custom Classes

If all you need is to add custom classes to body on a per page / post basis, this plugin is all you need.

Works nicely!

Allows users to create custom classes on a per-post/term basis.

nivijah

<p>This simple plugin adds two additional fields to the media window, that lets you set additional classes to images on top of the default ones.</p>
<p>After installation two fields will be added to the media window under an image, One for custom class for featured image, and the other for images who are added into the WordPress editor.</p>
<p>Multiple class should be seperated by “space”.</p>


Custom CSS classes for images

This simple plugin adds two additional fields to the media window, that lets you set additional classes to images on top of the default ones.

Custom Classes vs Custom CSS classes for images