For security reasons, the customer signature is embedded in the email in a special data format that gets converted to an image by the email client or web browser.
Unfortunately some email clients won't display this embedded image, such as the Google mail client, Gmail, running in a web browser.
For the Gmail client you should download the invoice/order from the attachment sent with the worksheet, you can then open the downloaded file in a browser and the signature will display and also show when printed.
Also see our FAQ Why is my logo or other images not displaying in emails?