The LULU Key Box uses the picture you uploaded to your own SL Profile page.
For best results, make sure your picture is 512 x 512 pixels. A size of 1024 x 1024 or 2048 x 2048 will look better when zoomed in but may delay load times.
Any other dimensions would trigger SL's own re-sizing process, which might not be optimal.
The photo on the LULU Key Box will only show if:
When you uncheck this checkbox, the photo will not show immediately. Typically you will have to wait (even a day or two) before SL makes the profile available.
Once the SL makes the profile available, just click the photo again to refresh.