Use full resolution image if available

The [`gallery.json`](https://www.planet.com/gallery.json) includes URLs for higher resolution images.  The extension now displays those images where available.

From where I sit, the load time is only marginally worth the wait.  I think it is worth looking into 1) preloading (this would only work for people who load multiple images from a single tab or would improve things if we had a deterministic load order) and 2) better compression.