Now I do realise that a PowerPoint presentation saved as a series of images can lose a lot in terms of animations, video, sound and transitions.
However do not compare the series of images to PowerPoint, but compare it to the learners not having access to anything.
A series of images may be better than nothing.
It should also be not seen as a series of images in isolation, the context would also be important and that series of images could be supplemented by an mp3 recording, a series of digital notes, or even a printed piece of paper.
Keynote (Mac presentation software) for example has the ability to save a presentation as a Quicktime movie which can then be converted into a range of mobile formats retaining, audio, video, animations and transitions.