Ten Things You Need to Know Today: Tuesday 18 Dec 2012

Users of the wildly popular photo sharing site Instagram are up in arms about a change to its privacy policy allowing it to sell users’s pictures to advertisers without notifying them. When Facebook paid $1 billion for Instagram in April many commentators suggested they had spent too much. Now it seems, the social media giant is trying to claw back some of that cash with controversial changes to its terms, the BBC speculates.

MPs and police officers have expressed concern at the arrest of the policeman – a member of the Diplomatic Protection Squad – suspected of leaking details of the "plebgate" altercation with Chief Whip Andrew Mitchell in Downing Street in September. They say the arrest raises chilling questions about the ability of whistleblowers to go public.

The Tories have lost one sixth of their support in the past two months and are down to 29%, their lowest ranking since David Cameron became leader. UKIP have reached a record high of 10% as voters take a harder line against the European Union, according to a Times/Populus poll, with 34% saying Britain would be "better off" out of the EU.

Arsenal have bounced back from last week's humiliation by Bradford with a 5-2 victory over Reading that included a hat-trick from Santi Cazorla. The win means they leap-frog Everton and West Bromwich into fifth place in the Premier League, and are only two points behind Chelsea in third place, easing the pressure on manager Arsene Wenger.