how do you add logos to email messages to make it look like letterhead?

If you mean in Yahoo Mail, on the toolbar just above the message box, there is a button for letterhead, however, you will find it requires Yahoo Mail Plus - for which there is a charge.

The Yahoo Mail toolbar gives us a number of goodies for free. You can change the color of the font, size, highlight, add a smiley --I think you will have to pay for luxuries - one way or the other - in dollars or SPAM.

I haven't seen that capability, yet, but you can add stationery and different fonts.

You can also create a Logo and then save it as a .jpeg and insert it as a picture in your email. I know in Outlook it would look like a Letterhead if you just insert it and then center it.

Good Luck