there are several ways to do this and everyone is welcome to share how they do this here
open email that shows your payment
click "print screen" (next to the F12 button)
open "ms paint" or your choice of program
hit "control V"
this should show your payment "edit and save"
then you have to use an image sharing site such as
http://photobucket.com/ or
http://my.imageshack.us/open this site browse your pictures and upload your payment get the code
copy and paste the code in your post