Step 1: Get the image you want to put on your button. The shipping Silver buttons use public domain images from the Open icon Library and you can find images of similar style there. You will probably want a 24x24 PNG. Save the file on disk.
Step 2: Drop a button from the palettes, e.g. the Silver OK button.
Step 3: Select the control and choose "Edit>>Customize Control" from the menu, or right-click and choose "Advanced>>Customize." A Control Editor window will open with the Control in it.
Step 4: From the menu, select "Window>>Show Parts Window."
Step 5: Right-click on the image you want to change. The Parts window shows which part you have selected, so you can verify that you have right-clicked on the right piece.
Step 6: Select "Import from File" in the right-click menu.
Step 7: Browse to the image file on disk.
Step 8: You're done! You can save the CTL or Apply Changes without saving to change the button in the VI.
Note: Silver style controls are only supported in LabVIEW 2011 and later.
Bonus: Do you want a button where the image stays centered instead of staying on the left side of the button? Use the attached CTL as your starting point and follow the same steps as above.