Sending an attachment o iMac

You could choose Share in Photos to open an email with a picture, and then you can add text.

Or, you can select a picture, choose more (...), and choose copy. And then you can paste the picture into the email where you want it.

You use the word "attachment," and each of these methods produces an attachment with the added information of where the picture can go in the email. I mention this, because some have asked about the inclusion vs an attachment, and they're the same. How they are displayed depends on the receiver's email system.