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.
Plug in your thumb drive to your Cr-48, and wait for the activity lights to quit flashing. Click on the “Upload a Photo” button on Picnik and you should see your thumb drive listed in the left panel of the upload dialog box. Click your drive name, and select your photo.
Make any edits you want to your picture, then select the “Save & Share” tab. Select PNG as the format, and click the “Save Photo” button.
In the left panel of the dialog box, click the chronos folder, and scroll all the way to the bottom. You will see a PNG file with your gmail address as its name (EDIT: If you did not take a picture when you set up your profile, this file will not be created, and I have not found a way to get Chrome to recognize a new profile picture even if you create a file with your email address as the file name. Even if you create the file, Chrome will not recognize the image as your profile picture.). Click that file, then click ok; confirm replace.
Log off; enjoy!