The Key Box photo uses the picture you uploaded to your own SL profile page.

For best results, make sure your picture is 512 × 512 pixels. A size of 1024 × 1024 will yield better results when zoomed in (but is probably unnecessary and would delay load times).

If you have Photoshop or a good image resizer, you might be better off resizing your pic to a square (either 512 × 512 pixels or if you must, 1024 × 1024 pixels), before you upload.

Any other dimensions would trigger SL’s own resizing process, which might not be optimal.