Opengraph images are usually heavily cached by social networks. Like @Port_of_Folio said it can take a couple days for them to update. Facebook has a dev tool that lets you cue it for rechecking/updates.
Twitter used to have one but it seems like have shut down the card preview outside of just the one you see when writing a tweet.
@sebarothmannb - The cache Austin was referring to is that of the respective social service. They cache for performance reasons. Each has its own way of letting you see what they have cached and potentially forcing a refresh if your graph changed.