NSURL *url = [NSURL URLWithString:@\"yourimage\"];
NSData *data = [NSData dataWithContentsOfURL:url];
UIImage *img = [[[UIImage alloc] initWithData:data] autorelease];
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You can do it with
UITableViewCell *cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"Cell"];
[[cell imageView] setImage:img];
But thats probably not going to run very well because you're downloading the images synchronously and that will most likely make your app stutter.
You should use lazy loading for the images, for an example take a look at http://developer.apple.com/library/ios/#samplecode/LazyTableImages/Introduction/Intro.html