Some event details are cached, including images, and thus do not appear on the site immediately after saving the updated event details. An event will update the cached details around fifteen minutes after an initial save. There is a way to "bust the cache" for events so that the server grabs the new details immediately. To bust the cache, you can change the title of the event and save the event. Typically, I will add a period to the end of the title, save it, then change it back and save it again. Doing this will force the event to update on the site and all the new event details will display as expected.
Cacheing is a standard practice for websites used to avoid excessive server data transfers. Left alone, our system will wait (sometimes up to 15 minutes) until the last change is made and then update event details. This is supposed to give users time to make all the changes they want and then update those event details on the server just once. This means a user can change things several times in an event and save the details over and over without forcing our servers to make multiple updates.