Softwares as Keynotes, PowerPoint or Impress are actually targeting the paper format, which is essentially 4:3 or around that. I assume by your requirement that you are trying to produce a presentation that will be shown on a HD TV.
This problem has been existing for a long time and if my memory is good, PowerPoint could be tweaked to support non-printable format. The issue was more with the content not always showing as expected.
Back to your problem.... Since you want to display pics and movies in some kind of presentation, iMovie is the tool to use. The drawback is that depending on the length of your presentation, it may take some time to produce. On the good side, iMovie is great when you need to update the content as a movie is kind of a project where the media can be easily modified and replaced.
You'll also benefit from great transitions, effects, titles and so on.
Of course, it depends on your presentation complexity to know if iMovie will really do the job.
Another benefit from videos instead of presentation documents is that some HD TV are able to play some movie formats directly from a USB key, meaning that there is no need for an external support. Just play the movie and make it loop if the presentation does not need to be interactive.
For a more geeky approach, web pages to hold you content may do the trick and are easily updatable. All you need is a server displaying the content in a full screen browser.
What would be interesting to know is what you are trying to produce and for what purpose as it all depends on this.
My 2 cents