There are email templates in iPhoto, to help you style your message. The placeholder text is there to show you an example of the message. If you click on the placeholder text, you can change it.The stmp.gmail.com keychain alert is just asking you to allow iPhoto to send the message. You may choose OK, or Yes.
If you haven't typed in any of your own text, iPhoto will send your photo without a message.
Yes it is ok to "allow".
That's a very different question, but no, if you email a photo to someone, you can't really prevent them from posting it somewhere. You can tell them they're not allowed to use it, but that's difficult to enforce.Yes, the receiver will only see the photo, or photos, you emailed to them.
You're welcome. Being able to fix computers quickly just comes with a lot of experience.You can learn a lot on this site: https://support.apple.com/videos/