Creating a Signature for your mails in Roundcube is quick and easy! I've found this really useful video that takes you through it step-by-step Here.
Alternatively you can follow these steps:
In order to add an image to your signature in RoundCube you will be required to:
1. Log into Roundcube at http://www.yourdomain.com/webmail/ (replace yourdomain.com with your domain name).
1. Select "Settings" on the top right hand corner.
2. Select Identities -> then select your identity/signature.
3. Select the "HTML signature" tick box.
4. Select the "HTML" icon in the WYSIWYG (What you see is what you get) editor.
5. Insert the following line of HTML:
make sure you replace the link above with a link to your own logo.
6. Select "Save".
7. Select "Preferences" in the left hand menu.
8. Select "Composing Messages"
9. Change "Compose HTML messages" to "always"
10. Select "Save".