Accessing data in firebase realtime database swift

I don't understand your question so I will write a code in order to retrieve the data from realtime database and to save it in an array.

You said "Each image is saved with a timestamp", so I assume you want to retrieve an array of data containing images(imageURL, etc) right?

we create a structure based on your database

struct dataStructure {
var Description:String?
var imageURL:String?
var Portfolio:String?
var createdAt:String?
var Instagram:String?
var profileImageUrl:String?
var Twitter:String?
var Username:String?
}
// now we create an array of this struct

var arrayOfData = [dataStructure]()

func downloadFromFirebase(completion:@escaping (Bool ,Error?)-> Void) {
      ref.child("ImageDetails/").observeSingleEvent(of: .value) { snapshot in
      guard let value = snapshot.value as? [[String:Any]] else { return }

// we add each element of value in the arrayOfData
        for element in value {
            guard let name = value["username"] as! String,
                  let Description = value["Description"] as! String, 
                  let imageURL = value["imageURL"] as! String, 
                  let Portfolio =  value["Portfolio"] as! String,
                  let creationDate = value["createdAt"] as! String, 
                  let Instagram = value["Instagram"] as! String, 
                  let profileImageUrl = value["profileImageUrl"] as! String, 
                  let Twitter = value["Twitter"] as! String , 
                  let Username = value["Username"] as! String else {
                   completion(false)
                   return
                     }

  arrayOfData.append(dataStructure(name: name, 
                                   Description: Description, 
                                   imageURL: imageURL,
                                   Portfolio: Portfolio ,
                                   creationDate:creationDate ,
                                   Instagram: Instagram, 
                                   profileImageUrl: profileImageUrl,
                                   Twitter: Twitter))

        }


        completion(true)
        }
}

at the end you will have an array with all your data

here is the code if you want to take the data from this function in your completion

func downloadFromFirebase(completion:@escaping (Result<dataStructure, Error>) {
      ref.child("ImageDetails/").observeSingleEvent(of: .value) { snapshot in
      guard let value = snapshot.value as? [[String:Any]] else { return }

// we add each element of value in the arrayOfData
        for element in value {
            guard let name = value["username"] as! String,
                  let Description = value["Description"] as! String, 
                  let imageURL = value["imageURL"] as! String, 
                  let Portfolio =  value["Portfolio"] as! String,
                  let creationDate = value["createdAt"] as! String, 
                  let Instagram = value["Instagram"] as! String, 
                  let profileImageUrl = value["profileImageUrl"] as! String, 
                  let Twitter = value["Twitter"] as! String , 
                  let Username = value["Username"] as! String else {
                   completion(.failure(error)
                   return
                     }

  arrayOfData.append(dataStructure(name: name, 
                                   Description: Description, 
                                   imageURL: imageURL,
                                   Portfolio: Portfolio ,
                                   creationDate:creationDate ,
                                   Instagram: Instagram, 
                                   profileImageUrl: profileImageUrl,
                                   Twitter: Twitter))

        }

       /* we provide the array with the new data in the completion at the 
          end of the loop */
        completion(.success(arrayOfData))
        }
}