How To Setting To Download The Image Of Products

Those products of my website with only one image by products, they have "save image as" when click on the right of the mouse, please try for those pages(login email:, password: 654321 to see the image):

But if the products have the additional images, they lost the "save image as" when click on the right button of the mouse, don't show anymore this function, please check those pages and try:
Does someone can show me how to set or modify to let the user can download all images please?
My website use multi vendor with 4.2.3.
Thank in advance!!!

In Chrome I can save images of all mentioned products.