If by hard drive picture you mean icon, then heres how you do it.
1) Find an icon set.
2) Select the one you want.
3) control click it and go to 'Get Info'
4) You should see a miniature icon in the top left corner. Click that. It should highlight (get a highlight aura around it).
5) Once its highlighted, press command-c (copy command; apple key held down followed by c).
6) Now select your existing hard drive icon on the desktop
7) Repeat steps 3 and 4 on the existing hard drive icon.
8) Once its highlighted, press command-v (paste command; apple key held down followed by v).
9) Enjoy.