Resize for new ad size

[grgar]

Per the Community Promotion Ads 2016 guidelines, I've resized the ad to 600 px × 500 px and added a 2 px border.

For Does anyone want to take over or suggest a fix to the bounty counting ad?.

[bmike]

Looks great. Thank you @grgarside

[kylecronin]

I pushed the update to Heroku