[e.cash] Revert twitter image url

Summary: As far as I can tell the outside image source did not help with the twitter preview image. reverting back e.cash

Test Plan:
Test the twitter preview as the site is now.
If its not working we can revert the url and try another approach

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14457