Click on "Attach a file"

Every email program has a way to attach a photo.  In this case it says "Attach a file", but a photo is just another sort of file, so that means photos too.  Notice the "Paper Clip".  For some reason, the picture of a paper clip has come to mean "Attachment".  I guess that is from the idea of paper clipping attachments to the main message.  In any case, you need to locate the "Attach" link in your specific email program.