You can edit the HTML or right click on the image and see where it is located. Create a new image and name it the same as the image that is on your site now. Then upload it to the same place so it overwrites the image.
First, clear your browser cache. When you are in the ACP, right click on the image and note the path. Then do the same when you see the x and see if that will lead you to know if they're in the right spot.