As far as I know, you can't "attach" photos to mails (I'm assuming that you're on an iPhone since you're posting in the iPhone forum). The only way to send a picture through mail is to open your Photos.app, select a picture, and press the "arrow going out of frame"-icon, then you'll get an option to mail the picture.
If you've already written some text in your reply, I suggest you copy that, so you can easily paste it into your new message.