Go to your Facebook Page and locate the App buttons.
Click the down arrow on the last button to expand the button area
When you put your mouse over the Look inside button, an
Edit icon appears. Click it and choose Edit Settings
To change the button text, enter a new text in the Custom Tab Name box and click Save.
To change the button image, click Change at Custom Tab Image.
In the new window, put your mouse over the image and click the Edit icon
Choose the new image file. It must be 111 x 74 pixels.