(@EmilyThornberry)

To readers outside of Britain, it may be hard to understand exactly why the tweet picture above could lead to a senior politician resigning. It probably seems completely innocuous, if not bland: A picture of a house, decorated with flags, with a white van parked outside. The photograph is accompanied with a very basic description. What exactly is there to get annoyed about?

However, it's this very picture, tweeted out just a few hours ago, that led the Labor party's shadow secretary general  to resign from the front lines of British politics. Emily Thornberry's resignation came on the same day of an important by-election.

To understand why the picture caused such outrage, you have to be able to read the coded class messages many thought were contained in the image. Thornberry may not have intended to send these messages, but class-conscious Britons found them anyway -- and right now, these messages might be more important than they have been in any time in recent years.

First, consider the flags in the photograph. Two of them are St. George's Crosses, the national flag of England. Another flag is that of West Ham, a professional soccer club based in East London. These flags may seem like nothing -- decorations for a patriotic sports fan -- but in the United Kingdom, they carry a lot of weight.

The English flag in particular has long been linked with far right political groups like the British National Party and the English Defense League, leading to a stigma that's been hard to shift: In 2012, one poll suggested 24 percent of English people considered the English flag to be racist. Then there's the West Ham flag. The football club is based in a working class area of East London, and at the height of Britain's football hooliganism era (the 1970s and 1980s) the club's fan base gained a reputation for violence and links to the far right. Things have changed, but that reputation has stuck: The terrible film "Green Street Hooligans" used West Ham's fans as a model, for instance.

The white van is also an especially potent symbol in the United Kingdom. The "white van man" is a well-known stereotype in the United Kingdom. The stereotype goes that the "white van man," who usually works in a trade like plumbing or electrical work (hence the van), is a particularly vocal member of the working class. While the term was occasionally a celebration (the tabloid newspaper The Sun ran a column where they asked a "white van man" his thoughts), it was also used as an insult and "white van men" were viewed with derision and contempt.

On an ordinary day, Thornberry's picture might not have meant much. But there's a wider context here. Thornberry had traveled to Rochester, a small town not too far from London, because there was an election that day, an election that had been triggered by the local MP Mark Reckless' shock defection to the upstart U.K. Independence Party in September. If Reckless wins this election, as he is widely expected to do, it will be UKIP's second seat in parliament, and could be a harbinger of more success to come.

Reckless' win would be a blow for Britain's established right wing party, the Conservatives. UKIP has wooed many Conservatives by pursuing a line further to the right: In particular, they take an especially tough line on membership of the European Union and immigration, appealing to the more conservative of the Conservatives.

However, it's also a problem for Labor. The left wing party was once seen as the voice of the British working class, but in recent years that relationship has been fractured. Many were affected economically by immigration and globalization, and felt that Britain was changing, but not in their favor. Since Tony Blair's election in 1997 and the re-birth of the party as "New Labor," the party's leadership was no longer associated with the unions and the working class: Now most of their top leaders are part of Britain's Oxbridge-educated political elite.

Emily Thornberry, who represents the well-to-do London seat of Islington South and Finsbury in parliament, would appear to be part of that elite. By tweeting out a photograph of a small house, with a bunch of symbols of the working class, it really looked a lot like she was trying to make some sort of statement. People noticed.

Political blog Guido Fawkes pointed to her $3 million home in Islington, which it noted was "just one of the many properties she owned." UKIP leader Nigel Farage asked what she was "trying to imply" about Rochester in a tweet. Tabloid newspaper The Sun later ran a headline that said she was "only here for the sneers."

Thornberry at first tried to deflect the criticism, telling reporters the furor had been caused by "prejudiced attitude towards Islington." Later, however, she apologized, taking to Twitter again to say sorry that there was nothing wrong with flying the England flag. However, it was too late.

"Betrays your real thoughts about the English working class you claim to represent," one Twitter user wrote in reply. "You despise us. We hate you."

Thornberry's resignation came just as the polls closed. The British press is reporting that she resigned after Labor Party leader Ed Miliband, hoping to win an election next year, made it clear he was "furious" about the tweet. "Earlier today I sent a tweet which has caused offence to some people," Thornberry, a close ally of Miliband, said in a statement. "That was never my intention and I have apologized. However, I will not let anything distract from Labor’s chance to win the coming general election. I have therefore tonight told Ed Miliband I will resign from the shadow cabinet.”

As my colleague Griff Witte noted today, the Rochester by-election was already a big deal -- a confirmation of UKIP as a force to be reckoned with, and a green light to any other British politicians thinking of making the jump to the upstart party should Reckless win. Now, no matter how ridiculous the scandal that brought it is, Thornberry's resignation has also confirmed just how easy it is for a politician to fall afoul of the shifting dynamics in Britain's political world.

For many people, however, the political intrigue was irrelevant. That apparently includes the white van-owning, England-loving West Ham fan at the center of the scandal. “I’ve not voted yet and I’m probably not going to vote,” the man, who owns the flags and the van pictured in the tweet, told Buzzfeed News.