First make sure your work is saved. The default file type is ".pdn" which has your layers but no one else can see it if they're not in Paint.net. So we need to save a copy.
- Go to File>Save as... (or CTRL+SHIFT+S)
- Change the file type to "JPG".
In your FreshGrade portfolio, click on the Upload button under the assignment... then browse to find the JPG file.