A couple of options for you:

- To create a new shelf icon image simply do a screengrab of Maya's viewport, cut the relevant bit out (making sure the size is 32x32 pixels). Now modify the icon in something like photoshop and save as a .bmp to a location where you can load it from. I usually put it in:
<drive>:/Program iles/Autodesk/Maya<version>/icons/<myiconfolder>/new_icon.bmp
or:
<drive>:\My Documents\maya\<version>\prefs\icons
but you can put them anywhere really

- To use an existing or newly created shelf icon go to Window -> Settings/Preferences -> Shelf Editor. A window pops up which allows you to choose shelves and shelf contents. Choose the shelf button you want to change the icon to and click the 'change image' button.
Now locate where the icon you want is (if using the first step above) and it will add it in.

- To copy a shelf item, first open up your script editor (Window - > General Editors -> Script Editor) and press the shelf item that you want to copy. You should see a Mel command(s) appear in the script editor. Highlight that code and middle mouse button drag it to your shelf. A popup will appear asking if you want it to be Mel or Python - choose Mel

Si