Well, even though it is fairly atypical, I agree with Cure's suggestion of
The Spartan. You could start off discussing how the pixels are comprised on red, green and blue components. Also, you could talk about how subtle differences in pixel placement can alter the viewers' perception of the image - in Helm's image there really does seem to be more than just red, green and blue, but of course there isn't! It's a good example because I'm sure your viewers will be wowed by how different it looks zoomed in.
Also, many people immediately think 'isometric' when they hear 'pixel art' (stuff like Habbo and Eboy spring to mind for many). A great example of an isometric piece would be
this one by Fool. You could talk about all the minuscule detail that he's placed into that image. You could show close-ups of some of the people and talk about how he's managed to breathe so much life into those characters even though they're so tiny.
You might also want to touch on some standalone sprites too. Personally, I think it would be best to talk about something like the Super Mario Bros. Mario sprite, considering how iconic it is. You should mention that even though it only has 3 colours (excluding transparency) and fits within a 16 x 16 pixel canvas it still manages to be incredibly distinctive. You can even start moving around pixels in the sprite to show what a difference single pixels can make.
Anyway, good luck. Hope some of this helped, it was pretty much me just rambling though
