Please browse to your Facebook fan page while being logged as an admin and then:
1. Click on 'Settings' (located top navigation bar)
2. Click on 'Edit Page'
3. Click on 'Settings' for Store and then click on 'Edit Settings'.
4. You may now upload a new image or edit the text.
We recommend on putting an image with a relevant call for action like "Shop now!".
Note: one needs to be a page admin to do so, so I can't do that for you.