It sounds like you saved an image instead of a project... so even if you give it a .wbi extension the file is being saved incorrectly:
When saving a project, click the save project button and give the filename a ".wbi" extension.
When saving as an image, click the "Save" button and give the filename a ".jpg" extension.
So again, click on the save project button to save projects and give the filename a ".wbi" extension. So if you want to name your project "myebookcover", name it "myebookcover.wbi".