Trouble Uploading Multiple Images to Firebase Storage

I have an array of image data(in bytes), and I want to upload them to storage and get the download URL for all of them. But after uploading lots of pictures, there is only one file appears in the storage under the reference. I do get the download URLs, but only the last one works, the rest all say access denied.

I suspect it is because I am putting all the pictures under the same storage reference and the picture substitutes every time. But what can I do to upload multiple images?

My upload code:

var picturesURL: [String] = [] // I want to put the download URL in this array
            
if self.imageButton.isSelected{
    picturesURL = []
    
    for photoData in self.pictures{ // pictures is the array that stores all the photo data
        self.storage.child("images/file.png").putData(photoData, metadata: nil) { _, error in
            guard error == nil else{
                print("Failed to upload")
                return
            }
            
            self.storage.child("images/file.png").downloadURL (completion: { url, error in
                guard let url = url, error == nil else{
                    print("problems getting url")
                    return
                }
                
                let urlString = url.absoluteString
                picturesURL.append(urlString)
                print(urlString)
                //UserDefaults.standard.set(urlString, forKey: "url")
            })
        }
    }