You have a built-in option in Classic Start Menu for setting a "Classic" button which accepts an ICON (*.ico) file for the image it shows. Select the Classic button and pick an icon you want.
Your image shows that you are looking for the XP Classic button but you mention the original Windows 95 button so I have attached both although these are custom images, not an icon for the Classic button. To set these, you need to choose "Custom Button" and then set the PNG.
In case by original, you mean the Windows XP Luna button here is it:
http://www.classicshell.net/forum/viewtopic.php?t=1152Although I could only create these for the small taskbar size. For the large taskbar size, it is recommended you choose the Classic button and pick an icon, not a PNG.