If GhostMail was compromised, the attacker would be able to read messages as they are being sent. Sending messages as images does not stop them from being copied or held for an indefinite amount of time.
In the unlikely scenario that the app was compromised and stayed compromised, you would be correct. However, if it was compromised and the situation was quickly rectified, only sent but unread messages would be able to be read. If a service like WhatsApp was compromised, however, a much larger amount of messages would be able to be read.