How to include original images or attachments in my email reply?

In Apple Mail, select the original message, then go to the "Message" menu and choose "Send Again". This will create a new message with the same content as the original, but being sent from your account. To avoid confusion, you should manually quote some of the message so the recipient can tell it is a reply. Select some of the original text and go to the "Format" menu. Choose "Quote level" > "Increase".

Another option is you want to be really fancy. Click the "Reply" button as normal. Select the quoted text from the new reply window and copy it to the clipboard. Close the reply. Do the above "Send Again" procedure and replace the original text with the copied reply content, including the "On yada, yada, yada date, really important person wrote:" and leave the attachments as is.