As you have realised, HTML is with all the fancy bits on, plain text is just that. Even though you cannot send pictures with the message, you can still send them as attachments.
Note: if your recipient has his machine set up as plain text, it doesn't matter how fancy and decorated you send your message - it will be received plain thus, you would be wasting all your efforts.
Most businesses are set to receive in plain text to lessen the possibility of virus transmission.
I always have mine as plain text and sent anything else as an attachment which can then, if necessary, be easily virus checked at the other end before opening.