Failed to load resource: the server responded with a status of 403 ()

Hi. I have a problem with showing some photos in wonderful image slider plugin. When i open the page with this slider it shows me slider with photos that are not showed. Instead of photos you can only see “undefined” and icon of a photo.
Also in inspector it shows “Failed to load resource: the server responded with a status of 403 ()” to each not loaded photo on site. Can you help me please?

Furthermore i’ve checked permissions and privacy, and this ype of data is publicly visible…