I remember that after we upload some picture, there is an icon under the picture that allow us to choose it as cover picture, in which it will shows as thumbnail picture for that listing and also shows it as the 1st picture.

I could not find that in 4.6.2? It seems that flynax pick the 1st picture to be cover picture, if I deleted it then the second picture will be the cover picture. So if I created new picture and want it to be cover picture, how can I do that without having to delete all previous pictures.