Always display initial image then replace with image-proxy one if it succeeds

[chrismytton]

When the page loads we should always show the anonymous person picture and then replace it with the one from the image proxy if the one from the image proxy successfully loads (which isn't always the case).

[tmtmtmtm]

It would be good if we used a slightly different anonymous person image, just so we can more easily tell the difference between "We don't even think we have an image for this person" and "We think we do, but we're in the process of loading it (or perhaps it failed)".