Why your email should not have only images:
Images are heavier than text, and the bigger they are, the slower it will be. Do not forget that more and more people check their emails on their mobile phone: limited bandwidth, longer load time, smaller screens…
On most email software, images are disabled by default. They will only be shown if the user clicks on the option to download images. At first glance, your email will look like a white square, with nothing to convince your reader that your message is of any interest.
On iPhone, images are resized to fill the available space on the screen. If your image also contains text, it's a safe bet to say it will be unreadable once shrunk. On Android, no resize happens: the larger images will just show scrollbars, making it difficult to read your whole message.
The receiving server is not able to understand the contents of an image. It may very well decide not to take any chances and simply block your email.