make sure that your insertion point is at the beginning of the text area of the email.
Photo's are inserted at the current cursor position (insertion point)
May solve your problem
I've created several signatures in Apple Mail Preferences to be used randomly and to appear beneath the text or image in an outgoing email. I've left the box empty next to "Place signature above quoted text." Process works OK with text, however, when sending a photo, the signature continually appears above the image. Is there some way to have the signature always appear at the end of the email when sending images or photos?