haha this is so easy to pull off. i do it in outlook all of the time. Forward the email to him/her and modify the time, date, and recipient of the original email in the text before you forward it. problem solved. Just make sure the font, color, etc. is all the same as the original.
tell them you don't know why they didn't get it the first time. most will look at the time and date sent of the original email and think nothing more of it. child's play. plus, this way you don't even have to mess with photoshopping anything and going to show them.