Replace placeholder image for current news items

[krivard]

#535 added support for news item images, but they're currently all set to a placeholder. We don't necessarily need a different image for each news item, just the most recent ones that will show up on the frontpage. The rest are optional maybe? cc @ryantibs

• 2021_05_12_spaig_award.md
• 2020_12_chng_partnership.md
• 2020_10_fellows.md
• 2020_04_covidcast.md
• 2020_03_covid.md
• 2019_10_CenterOfExcellence.md
• 2018_11.md (Delphi Wins 2017-2018 Forecasting Challenge)
• 2017_10.md (Delphi Wins 2016-2017 Forecasting Challenge)
• 2016_12.md (Delphi Wins 2015-2016 Forecasting Challenge)

[ryantibs]

Sure, relative priority here would be to fix the ones that appear on the front page definitely.

Since this wouldn't be too hard (I imagine many times it's just taking a screenshot of the relevant news posting), we could still do the others for completeness, eventually.