You can add attachments at any time while you're composing an email.
Here's how to do it:
Nice work. Look at you—adding attachments!
You can send many types of files as attachments, including word processor or spreadsheet documents, audio files, image files (bmp, jpg., gif, etc.), web pages saved as HTML files, and more. Your maximum total message size is 10MB.
If you'd like to be able to send and receive larger messages with attachments, check out Mail Plus. With Yahoo! Mail Plus, you can send and receive messages up to 20MB in size.