The focus (initially highlighted field) in the "New Image" dialog should be on the Width text field (the first field) rather than on the Height field. So it's faster to create a new image with a canvas size other than the default size.
This patch corrects that.
The ZIP-file contains a NIB-file, which has to be moved into:
To get into the Seashore.app package, Control-click the Seashore icon and click "Show Package Contents".