MB MB

This plugin makes it easy to display the featured images of your posts in mailchimp RSS newsletters.
Once activated this plugin will add the following to your sites default rss feed:
To the rss name space declarations:
<pre><code> xmlns:media="http://search.yahoo.com/mrss/"
</code></pre>
To each individual item (if it has a featured image):
<pre><code><media:content url="[url]" type="[mimetype]" medium="image" width="100%" height="auto">
 <media:description type="plain"><![CDATA[title]]></media:description>
</media:content>
</code></pre>
The featured images are not inserted directly into the content of your posts, so you have full control over where and how to present them. 
If you send out newsletters using a Mailchimp RSS based Campaign, simply place the *|RSSITEM:IMAGE|* merge-tag in your mail template where you want the featured image inserted.
This plugin contains
<ul>
<li>100% clean lightweight code.</li>
<li>0% JS</li>
<li>0% CSS</li>
<li>0% options</li>
</ul>
The plugin uses the hooks ‘rss2_ns’ and ‘rss2_item’, part of the WordPress core API. 
Tested with WP 3.7.1 and MailChimp Dec 2013
Contribute via <a href="https://github.com/mikkelbreum/MB-ImageChimp-RSS-Feed-Enhancer/" title="MB-ImageChimp-RSS-Feed-Enhancer" rel="nofollow ugc">github</a>

MB ImageChimp RSS Feed Enhancer

Adds featured images to the default RSS feed for use with MailChimps image merge-tag

Mailchimp

Use the Mailchimp List Subscribe plugin to quickly add a Mailchimp signup form block, widget, or shortcode to your WordPress site.
After installation, if you already have a Mailchimp account, you’ll log in with that account and then proceed to configure settings.
If you don’t have an account, you can create one directly in the plugin. After entering in all your personal details, you’ll need to activate your account via an email that will be sent to you. Once done, you’ll proceed to configure settings.
On the settings screen, you’ll select your Mailchimp list, choose merge fields and groups, and configure other options. Once done, you can now add the block, widget, or shortcode (<code>[mailchimpsf_form]</code>) to your site. Typically, installation and setup will take about 5-10 minutes, and absolutely everything can be done via the WordPress Setting GUI, with no file editing at all.
WordPress.com compatibility is limited to Business tier users only. <a href="https://mailchimp.com/help/ways-to-add-a-signup-form-in-wordpress/" rel="nofollow ugc">How to add a signup form if you have a WordPress.com site</a>.
<h3>Access Token Encryption</h3>
Starting in version 1.6.0, authentication has changed to use OAuth. As part of this process, we retrieve an access token that can be used to make API requests. To provide a high-level of security, this access token is encrypted before being stored in the WordPress database. In order to ensure this access token can be decrypted when used, the plugin relies on certain security constants that should remain unchanged.
With no additional configuration, we use the standard <code>LOGGED_IN_KEY</code> and <code>LOGGED_IN_SALT</code> constants that are normally set in your site’s <code>wp-config.php</code> file. Some sites make use of security plugins that rotate these constants on a periodic basis. When this happens, we won’t be able to decrypt the access token and you’ll need to reconnect your Mailchimp account to generate a new access token.
To prevent such issues, it is recommended to define two additional constants in your site’s <code>wp-config.php</code> file: <code>MAILCHIMP_SF_ENCRYPTION_KEY</code> and <code>MAILCHIMP_SF_ENCRYPTION_SALT</code>. These constants should consist of a combination of characters, preferably at least 32 characters long. Once set, these values should not be changed. For strong values, you can copy some of the values from <a href="https://api.wordpress.org/secret-key/1.1/salt/" rel="nofollow ugc">here</a> and use them. You’ll end up with additional code like the following in your <code>wp-config.php</code> file:
<pre><code>define( 'MAILCHIMP_SF_ENCRYPTION_KEY', 'put your unique phrase here' );
define( 'MAILCHIMP_SF_ENCRYPTION_SALT', 'put your unique phrase here' );
</code></pre>
If these constants are added after you’ve already authenticated with Mailchimp, you will need to reconnect your account. To avoid this, you can copy the values from <code>LOGGED_IN_KEY</code> and <code>LOGGED_IN_SALT</code> (if they exist) to <code>MAILCHIMP_SF_ENCRYPTION_KEY</code> and <code>MAILCHIMP_SF_ENCRYPTION_SALT</code> respectively.
<h3>Upgrading</h3>
If you are upgrading to version 1.2.1 and you used the widget in your sidebar previously, all you need to do is drag the <code>Mailchimp Widget</code> back into the sidebar, visit the Mailchimp settings page (which will have maintained your prior settings), click the “Update List” button, and you’re done!

Mailchimp List Subscribe Form

Add a Mailchimp signup form block, widget, or shortcode to your WordPress site.

MB ImageChimp RSS Feed Enhancer vs Mailchimp List Subscribe Form