When trying to add images or other rich content to messages, it becomes useful to enable the HTML editor; and when sending messages to older clients or over-active spam filters, plaintext messages are desirable. Changing the default can be done in RoundCube's settings.
This is also applicable if your signature contains images. The plaintext editor will strip these images from your signature.
Procedure
For One User
Log in to RoundCube
Open the Settings page
Select the "Composing Messages" settings tab
Change the "Compose HTML Messages" to the desired setting
Save your changes
For All Users
Log into the server via SSH
Edit the following file
nano /usr/local/cpanel/base/3rdparty/roundcube/config/config.inc.php
// compose html formatted messages by default
// 0 - never,
// 1 - always,
// 2 - on reply to HTML message,
// 3 - on forward or reply to HTML message
// 4 - always, except when replying to plain text message
$config['htmleditor'] = 1;