I have uploaded 4 images to test this. I am not sure what the result is you expect but maybe these directions help:
FileNames["/img/*", {"."}]]
This results in more than 4 answers. A file .gif can also have a .lock version. So the numbers left and right of your rule are not the same. The {"."} is necessary to search in the img directory and not deeper.
Use
CloudObjectInformation[#,"Name"]& /@ CloudObjects["/img"]
to get a list without double entries.
To read/import the in my case 4 picture files you can use:
Map[Import[#] &, CloudObjects["/img"]]
CloudObjects[] can be used in the web-interface.