You can attach a picture to any message if you select the "Attachment" icon on your Send a Message page:

Each MMS message counts as 3 messages sent per recipient.

If using the mobile app

If using the mobile app, you can select the "MMS" option in the "Type" section of the "Send a Message" page.

This converts your message into an MMS, which also increases your maximum character count to 1550 instead of 160. (Note that each MMS counts as 3 messages per recipient instead of 1, and MMS will be sent to your contacts through a different number than regular messages.)

Then tap the "choose file" box to select an image from your phone to send with your message.

Note: There is a size limit imposed of 0.51 MB from the mobile carriers and some devices do not accept PDF type files.