I am displaying some text and images in a UITableView. The image first gets downloaded. Since before the image gets downloaded, I don't know the size of image, so I initially put a UIImageView of some fixed size. And when the image is downloaded, I resize the UIImageView.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

// Download image

    dispatch_async(dispatch_get_main_queue(), ^{

    // UIImageView resizing  
    });  
});

All this happens in cellForRowAtIndexPath.
The issues I am facing here are :
1. How do I update the height of cell ? Considering that there can be many images in a single cell. So I need to change the position of bottom image when above one downloads.
2. I tried using UITableView beginUpdates and endUpdates, but that scrolls to the top of cell giving a poor user experience.

This is how the UI looks like on reloadData. There are 5 images to be downloaded : UI experience after UITableView reloadData

Short Answer

Give enough breathing space to estimatedRowHeight
Changing a UITableViewCell once returned by dequeueReusableCellWithIdentifier will not work with cached cells
Trigger a single cell reload with reloadRowsAtIndexPaths
Manage your cache with Core Data and let NSFetchedResultsController boilerplate code can do all the UI work.

In Details

No unexpected scroll, only updates images as they come:

If the cell being refreshed is at or below the horizon, the UITableView will not scroll
if the cell being refreshed is above the top, the UITableView will not scroll
the UITableView will only scroll when the cell is in plain sight, and requires more space than available.

Let UITableViewAutomaticDimension do the hard work

You need to tell Cocoa Touch that the cell is stale, so that it will trigger a new dequeueReusableCellWithIdentifier, to which you are going to return a cell with the proper height.
Short of reloading the entire table view or one of its section, and assuming that your indexes are stable, invoke -tableView:reloadRows:at:with: passing the indexPath of the cell that just changed, and a .fade animation.

Code:

override func viewDidLoad() {
    super.viewDidLoad()
    tableView.estimatedRowHeight = 250 // match your tallest cell
    tableView.rowHeight = UITableViewAutomaticDimension
}

Use URLSession. When an image becomes available, fire reloadRows:at:with:

func loadImage(_ url: URL, indexPath: IndexPath) {
    let downloadTask:URLSessionDownloadTask =
        URLSession.shared.downloadTask(with: url, completionHandler: {
        (location: URL?, response: URLResponse?, error: Error?) -> Void in
        if let location = location {
            if let data:Data = try? Data(contentsOf: location) {
                if let image:UIImage = UIImage(data: data) {
                    self.cachedImages[indexPath.row] = image // Save into the cache
                    DispatchQueue.main.async(execute: { () -> Void in
                        self.tableView.beginUpdates()
                        self.tableView.reloadRows(
                            at: [indexPath],
                            with: .fade)
                        self.tableView.endUpdates()
                    })
                }
            }
        }
    })
    downloadTask.resume()
}

Once in the cache, cellForRow merely reads into it from the UI thread:

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell
{
    let cell = tableView.dequeueReusableCell(withIdentifier: "id") as! CustomCell
    cell.imageView.image = cachedImages[indexPath.row]      // Read from the cache
    return cell
}

Example: fetch a random set of images from *Wikipedia*