[e.cash] Add epos.cash to use-ecash page

Summary: Adding epos logo to image folder and ePOS copy to use-ecash page

Test Plan: visit e.cash/use-ecash and check copy and link to ePOS at the bottom of the list.

Reviewers: #bitcoin_abc, alitayin

Reviewed By: alitayin

Subscribers: johnkuney

Differential Revision: https://reviews.bitcoinabc.org/D18450