WASHINGTON — They posed grinning beside a bust of George Washington and stood solemnly in front of a portrait of John F. Kennedy. A few even caught President Obama as he walked across the South Lawn to Marine One.

For visitors to the White House, Wednesday proved cause for rejoicing — and a post or two on Instagram — as Michelle Obama announced the end of a 40-year-old ban on photography during public tours. The first lady revealed the news in a video posted early Wednesday in which she literally tore up a sign bearing the rule.

The move, which drew near universal praise, is likely to generate a stream of positive images for the White House and the first family, especially among young people using social media.

Robert Dallek, a presidential historian, said the change would bring greater transparency and openness to the White House, particularly at a time when Republican presidential candidates looking to position themselves as Washington outsiders have criticized the administration for being out of touch.