Self-host Dr. Kirsten Gorman’s picture of sea ice (#1627)

* Self-host Dr. Kirsten Gorman’s picture of sea ice
I've reduced the image from 1.5MB to 106K

* relative url

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Co-authored-by: Mike Bostock <[email protected]>

Author: Fil

docs/marks/image.md

@@ -122,7 +122,7 @@ Plot.plot({
   style: {
     padding: "10px",
     color: "black",
-    background: `url(https://i.imgur.com/ru3KGWJ.png)`,
+    background: "url(../sea-ice.jpg)",
     backgroundSize: "cover"
   },
   marks: [