Because VoiceThread can zoom in and out on media, we recommend that your images, slides, etc. be at least 1280×960 (i.e., a 4:3 ratio). If they’re smaller than that, it’s likely they’ll be pixelated and blurry.
If you use a different ratio (e.g., 16:9), your media will be automatically cropped by VoiceThread or have black bands at its top/bottom. If your media has a portrait rather than landscape orientation, you’ll see black bands on its right and left sides.