With recent Spectacle (version 4.6.1), the image paths are now /dist/... rather than ./dist/... (as they were in ^1.0.0, which I guess is the last time I published a Spectacle deck to GitHub pages... 😮 ) This path change resulted in all broken images when I deployed via GitHub pages.
I fixed manually by changing directly in the bundle(!) (because (a) slide deck, not production software, and (b) lazy):

Perhaps something to think about as a configuration option for next version? Or I could open a PR on the docs to add info/a warning about this if you like.