To upload a file, access your MyCIC account and your spousal sponsorship application in the normal way (i.e. click on the 'Check status and messages' blue button. This displays the tracking stages page.
Then click on the 'View submitted application' blue button towards the bottom of the screen. This displays the 'Documents submitted by client' screen. In this screen should be a request for additional information from the visa office with a facility / button allowing you to upload images (JPG's) or PDF files.