NSURLSession - Downloading a small PNG image in the background

Can you help me please. I would like to improve this code, possibly by utilizing NSURLSession? How would I change this code to use this construct?

The objective of the code is to load a small .png image in the background.

NSDate *myDate = (NSDate *)[[NSUserDefaults standardUserDefaults] objectForKey:@"LastUpdate"];

NSString *img=[NSString stringWithFormat:phpLinkgetUpdates, myDate];
NSURL *url = [NSURL URLWithString:[img stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]];
NSData *data = [[NSData alloc] initWithContentsOfURL:url];
UIImage *tmpImage = [[UIImage alloc] initWithData:data];
imgUpdate.image = tmpImage;

Thanks for your help!