This is very much like how you upload your author photo.
To upload your book cover image to your profile, sign in, go to "My NaNoWriMo" ---> "Edit Novel Info."
If you scroll down, you will see an "Upload Novel Cover" box. Hit the "Browse" button, find your picture on your computer, then hit "Upload."
Photos must by 100x100 pixels or smaller at 72dpi, and must be under 50KB.
You can use any photo editing program (even the Paint program built into Windows) to resize your image.
