Displaying images from server in a table cell in a thread

We mostly use this concept when we need to load many images from server .Using this concept, the main Thread does not gets blocked till each image is loaded & user can interact with the UI. However, threading is used when you don't want your main thread to block till your aspected task is completed. 

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{

UIImageView *imgView= (UIImageView *)[cell.contentView viewWithTag:IMAGE_TAG];

id object = [imageDict objectForKey:[NSString stringWithFormat:@"%i,%i", indexPath.section, indexPath.row]];

if(!object){

[NSThread detachNewThreadSelector:@selector(displayingSmallImage:) toTarget:self withObject:indexPath];

}

else

{

if(![object isKindOfClass:[NSNull class]]){

UIImage *img = (UIImage *)object;

imgView.image = img;

}

}

}

imageDict is a global NSMutableDictionary

- (void) displayingSmallImage:(NSIndexPath *)indexPath{

    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    NSString *imageUrl = [url to be used for fetching image from the server];

imageUrl = [(NSString *)CFURLCreateStringByAddingPercentEscapes(nil, (CFStringRef)imageUrl, NULL, NULL, kCFStringEncodingUTF8)autorelease];

NSURL *url = [NSURL URLWithString:imageUrl];

UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:url]];

if(image){

[imageDict setObject:image forKey:[NSString stringWithFormat:@"%i,%i",indexPath.section,indexPath.row]];

}

else{

[imageDict setObject:[NSNull null] forKey:[NSString stringWithFormat:@"%i,%i",indexPath.section,indexPath.row]];

}

[self performSelectorOnMainThread:@selector(setImageInCell:) withObject:indexPath waitUntilDone:NO];

[pool release];

}

- (void)setImageInCell:(NSIndexPath *)indexPath{

[tableView_meals reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationNone];

}