A previous version of Yahoo Mail included stationery templates to add visual flair to your email. While the current version of Yahoo Mail no longer has that feature, it supports adding images to emails. This is enough to create your own email stationery on Yahoo Mail.
Copy and Paste Approach
You can paste any image into the email message body as long as you copy the image directly from your Web browser. In Google Chrome, do this by right-clicking the image in your browser screen and clicking "Copy Image." If you want to use an image on your computer, you must first upload it to a free public image-hosting service such as Imgur, Flickr or Photobucket; then copy the image from your browser and paste it into the email.
You cannot use a pasted image as a background, which eliminates the possibility of entering text over the image. While you can add text next to the image, you cannot align the text to the top of the image. Pasted images in the email cannot be resized, so you should size them using an image editor like Microsoft Paint before uploading them to a public image-hosting service. If your emails are formatted as plain text, you can't paste an image into the email. Switch to rich text by clicking the pair of arrows below the Yahoo Mail compose text box to solve this problem.
Ideal Image Size for Stationery
Paste wide images intended to function as headers or footers with a height of not more than 150 pixels. Images with a width exceeding 1069 pixels can be pasted but the excess pixels are cropped out.