You need to replace the Finder.png or Finder@2x.png files inside the Dock app package resources. Then in order for the Finder icon change to take effect you need to restart your mac in safemode (hold shift at the white screen when booting up). Once booted into safemode the icon change will finally take effect. You can boot into standard mode from then on.
The icon is originally from an icon pack named Maji, made my the artist Jonas Rask. It appears that his website is no longer active, so here is a quick link to a zip file containing the trash icons. [link]