How can you send mail using IMAP?

IMAP was designed to receive email messages, not to send it. There is no IMAP command for sending email AFAIK. There is, however, at least one IMAP server which supports a special 'Outbox' folder. When you place the message into this folder it will be sent automatically.

Check Courier-IMAP documentation on Sending mail via an IMAP connection. Note, that this is a non standard method and I'm not aware of any other server which supports this.

There RFC 4468 which extends SMTP so it can fetch the mail content from the IMAP server, but I don't know about any working and widely used implementation.

Talking about gmail: sticking with SMTP is probably the safest way to go.

IMAP is a mailbox protocol. It does not (natively) support sending mail, only accessing it. In order to send mail you must use SMTP. Its possible that there is an IMAP extension for sending mail, and its possible that Google Mail supports that extension, but I doubt it. Hence, if you want to send an email with attachments, you must actually have the full content of the message available to you to send.

By the way, now that any modern mail client (including the webbased ones) supports a Sent folder, you typicaly have to use both SMTP and IMAP to send a single mail. And there's a race condition between sending the e-mail over SMTP and successfully saving the e-mail to the IMAP Sent folder. Using IMAP for sending e-mail is a way to avoid this race condition.