Change ISCO org screenshot

ISC currently don't have an accurate screenshot (it's of the Security
Services website) which potentially compromises their image as
impartial. They have supplied a new screenshot image in
https://govuk.zendesk.com/agent/tickets/856092 and this updates the
image to the one they supplied (scaled/cropped to 287x215 as per the
other images in this folder)
Merge pull request #1822 from alphagov/change-isc-screenshot

Change ISCO org screenshot