DETROIT —Mitt Romney set out on Friday to deliver a sweeping and sober vision for how to revive the American economy in a major policy speech here. In the end, he delivered something else as well: an unintended lesson about how poor visuals and errant words can derail a candidate’s message in this modern political news culture.

In an unusual choice, Mr. Romney gave his speech inside Ford Field, a cavernous indoor football stadium with 65,000 seats.

To the television audience, it appeared perfectly normal. Mr. Romney could be seen standing at a lectern in front of a backdrop that had the logo of the Detroit Economic Club, the event’s host. And when the stadium audience of about 1,200 people clapped, they filled the screen as cameras panned across them.

But in the age of Twitter and the Internet, that is not all that matters.

Before Mr. Romney had uttered a word, reporters began posting pictures online showing the stadium from every available angle — almost empty, except for the chairs set up on the field itself, near the 20-yard line.