A pixelated picture consists of pixels you can actually see, especially large ones, so the image looks blocky and unclear. That can be on purpose — to hide a face on the news — or it can just mean the photo is low-quality or zoomed in too far. It is the usual adjective, and the past form, from the verb pixelate. British spelling sometimes doubles the l: pixellated. Though actually, there is an older, different word pixilated, from pixie, meaning slightly mad or drunk. People mix the two spellings. If you mean the computer-picture look, pixelated with an e is the one you want.