Introduction
Below are the steps to adjust the size limit for an image attached to a signature or an identity.
Procedure
From SSH
How to log into server using SSH?
Login to your server and edit the following file with your favorite text editor.
How to edit a file on command-line
/usr/local/cpanel/base/3rdparty/roundcube/config/config.inc.php
And look for the line containing the following information
// Maximum size of uploaded image in kilobytes
// Images (in html signatures) are stored in database as data URIs
$config['identity_image_size'] = 64;
This is around line 567.
Increasing this value will allow for larger image sizes to be attached to identities and signatures.