Changing Mail icon?

My mom just got a new macbook and she wants to have a new icon for the mail icon. I've changed it to a flowe that she wants, but it doesn't always stay that way. Sometimes when I open mail it changes back to the bird stamp, and when I close it it changes back to the flower. I've gone to info, copy and pasted the new icon, etc. Does anyone know how to get it to stay with the new icon all the time?

on a side note. how do u change the icons on the dock? and is it possible to change the icons of stacks?
thanks

If you right-click (or CTRL-Click) on the mail.app file, select the option to "Show Package Contents". From there, you'll have to navigate the folders to find the actual icon file that Mail uses. Backup the existing icon file and replace it with one of your choice (be VERY SURE you are replacing it with the correct icon format... .ico vs icn... I don't know offhand what it is and can't check at moment). This should do it.