Adding an image to a text message requires that you first add a photo to your appointment, and enabling the text messaging feature. 

On your appointment, click the envelope icon below the image you wish to share with the client via text message:

Next, add optional commentary to be sent as part of the MMS:

Click "Share" button to send out the MMS. This results in the following experience for your client:

As illustrated above, your client receives the image and the (optional) text you've added as part of the MMS message.