Flickr officials have no doubt that China is blocking their photo sharing site's images and the company is trying hard to figure out why this is happening and how it can be resolved.

"We're still investigating. There isn't an '800' number you can call to register your complaint," said Flickr co-founder and General Manager Stewart Butterfield.

For now, the block, apparently implemented on Thursday of last week, remains. Butterfield's team has ruled out all possibilities that the problem is technical in nature.

In the meantime, Flickr, a Yahoo unit, is tapping its contacts in Hong Kong and mainland China through Yahoo Hong Kong and "We're seeing what we can do to help get rid of the block," he said.

Butterfield is confident that the situation will be temporary because the site itself is accessible in China but without the images. "If they were serious about any kind of permanent block I think they would block the whole site and not just the images," he said.

He speculates that the reason behind the block is the discovery of images the Chinese government found controversial. "The easiest way for them to not have them served in China probably was to block all images," he said.

It's a peculiar position for a Yahoo unit to find itself in. Yahoo and other internet companies like Google have been sharply criticised for self-censoring their search engine results in order to stay on the Chinese government's good side.

Human rights groups have also taken Yahoo to task over the company's decision to turn over to the Chinese government information on members' internet activities, like email messages, that have played a part in the conviction and incarceration of journalists and dissidents.

In their defence, Yahoo, Google and the other criticised Internet companies have said they must comply with the local laws of the countries they operate in.

The issue will come up today at Yahoo's annual shareholders meeting. Shareholders will vote on a resolution proposed by the New York City Pension Funds calling for the company's management to implement policies to protect freedom of access to the internet across the globe.

Yahoo had requested with the US Securities and Exchange Commission that the proposal be excluded from the shareholder proxy statement, but the agency declined the Yahoo’s petition.

The New York City Pension Funds, which include the city's Police Department and Fire Department funds, inserted a similar resolution in Google's shareholders meeting, held in May. The resolution didn't pass.

The organisation also put a similar resolution in Microsoft's shareholders meeting, which will be held later this year.

As of last Friday, the funds had about $338m in Google shares, $870m in Microsoft shares and $124m in Yahoo shares.