While WP E-Signature does not store images on your ftp server, it does use temporary imaging technology to convert the typed or drawn signatures temporarily so they can be displayed on in the web browser when you (or your signers) are viewing their signed documents.

The GD Library is a server side library that WP E-Signature uses to create and manipulate these temporary image files in a variety of different image formats, including GIF, PNG, JPEG, WBMP, and XPM.

If it has not yet been enabled, you will likely need to contact your website host and request they enable this feature on your website.

Read more about how to setup and configure the GD Library here:
http://php.net/manual/en/intro.image.php