BEIJING—China’s already formidable internet censors have demonstrated a new strength—the ability to delete images in one-on-one chats as they are being transmitted, making them disappear before receivers see them.

The ability is part of a broader technology push by Beijing’s censors to step up surveillance and get ahead of activists and others communicating online in China.

Displays of this new image-filtering capability kicked into high gear last week as Chinese dissident Liu Xiaobolay dying from liver cancer and politically minded Chinese tried to pay tribute to him, according to activists and a new research report.

Wu Yangwei, a friend of the long-jailed Nobel Peace Prize laureate, said he used popular messaging app WeChat to send friends a photo of a haggard Mr. Liu embracing his wife. Mr. Wu believed the transmissions were successful, but he said his friends never saw them.

“Sometimes you can get around censors by rotating the photo,” said Mr. Wu, a writer better known by his pen name, Ye Du. “But that doesn’t always work.”