Scrolling is very choppy in UITableView when loading images - ios

Question

I am creating an application that Holds about 26 images inside of a structure, and then I load those images into a table view, and anytime a new image comes into the View of the simulator, there is a bit of lag. It is definitely a problem dealing with loading images inside of the Table View.
I think I need to add an async, but I have no idea how to do that. If someone could guide me in the right direction on what I should do to achieve smooth scrolling, that would be awesome!
I wish I could post a photo, but I don't have a good enough reputation on StackOverFlow.com.
override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = self.tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as UITableViewCell
var question : Question
question = Questions[indexPath.row]
var titleLabel = cell.contentView.viewWithTag(1) as UILabel
titleLabel.text = question.name
var backgroundImage = cell.contentView.viewWithTag(5) as UIImageView
backgroundImage.image = UIImage(named: question.pic)
var frame : CGRect = CGRectMake(0, backgroundImage.frame.size.height - 200, backgroundImage.frame.size.width, backgroundImage.frame.size.height - 200)
cell.layer.shouldRasterize = true
cell.layer.rasterizationScale = UIScreen.mainScreen().scale
return cell
}

Answer 1

It has to do with the size of the images. Use a smaller thumbnails for the tableView cells. Convert your images smaller for it.
CGSize destinationSize = CGSizeMake(160, 160);
UIGraphicsBeginImageContext(destinationSize);
[originalImage drawInRect:CGRectMake(0,0,destinationSize.width,destinationSize.height)];
UIImage *thumbnailImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Answer 2

Kreuzberg's answer solved my problem, as I was getting jerky performance even with GCD and main_queue implemented. Since I can't add too much code in comments, I expanded on his answer so the width/height could be a bit more dynamic.
if (image.size.width > MAX_THUMBNAIL_RESOLUTION || image.size.height > MAX_THUMBNAIL_RESOLUTION) {
int width;
int height;
CGFloat ratio = image.size.width/image.size.height;
if (ratio > 1) {
width = MAX_THUMBNAIL_RESOLUTION;
height = width / ratio;
}
else {
height = MAX_THUMBNAIL_RESOLUTION;
width = height * ratio;
}
CGSize destinationSize = CGSizeMake(width, height);
UIGraphicsBeginImageContext(destinationSize);
[image drawInRect:CGRectMake(0,0,destinationSize.width,destinationSize.height)];
image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
}
You can make the constant MAX_THUMBNAIL_RESOLUTION to be anything you think reasonable. Or, you can calculate screen density and use a multiplier.