create & use all-years image

[7yl4r]

The image for all years should be the defaul that shows up when the page opens, and then decades can be toggled through.

[MathewBiddle]

done in

• https://github.com/MathewBiddle/globe/commit/ac17004b2dc8d5b60ce23d96595a97548f06a93f