To review the email inviting the client to complete the questionnaire, go to Account > Settings in the main menu bar. Click on 'Email Settings' and review the reply to email address, the from name and add your desired footer text. Click on 'Email - Invite to Questionnaire' to edit the body of the email as well as the subject line as shown below:
The actionable (and therefore important) part of the email, is the last line and button, so don't change this part.
Use the formatting bar to edit and make changes as you would in a normal document. To add an image or a link, simply click on the buttons shown below:
When you're finished, don't forget to scroll down to 'Save' your changes.