static NSString *CellIdentifier = @"MyTableViewCell";

MyTableViewCell *cell = (MyTableViewCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
    NSArray *topLevelObjects = [[NSBundle mainBundle] loadNibNamed:CellIdentifier owner:self options:nil];
    cell = [topLevelObjects objectAtIndex:0];
    cell.previewImage.layer.cornerRadius = 20; //Made it 20 to make sure it's obvious.
}

#import <QuartzCore/QuartzCore.h>

cell.previewImage.layer.masksToBounds = YES;
cell.previewImage.layer.opaque = NO;

cell.previewImage.clipsToBounds = YES;

cell.previewImage.layer.cornerRadius = 20;