Adding screenshots from the class' lectures to the course notes is a nice way to contribute. Udacity's Wiki has the capability of hosting images. Use the image uploader to upload images (diagrams or screenshots from the class, whatever you need to improve the course notes or any other Wiki content):
Click the link and the following screen should open:
Choose an image from your computer:
And then click "Upload". The following screen will show:
Now to add this image in your wiki page, just copy one of the Wiki Codes (there are three available sizes) and paste it in your Wiki page.