As other sites have mentioned, it’s not possible to replace your initial profile picture on the Cr-48. Until now. The easiest way to begin this process is by copying your intended pic to a thumb drive from your primary machine. On your Cr-48, install the Picnik app, then run the app.
I just received my Cr-48 in mail, and as per usual Google products, this one is not free from dry humor. The safety notices were no exception and were funny enough to share: