This should do the trick. It don't think that your check for NSNull is not the proper technique to check for a null. Try one of the checks in the first if
statement:
/* Either of these checks in the if statement should work */
if ((NSNull *)imageUrlString != [NSNull null] || [@"<null>" isEqualToString:imageUrlString]) {
imageUrlString = [userImageArray objectAtIndex:indexPath.row];
} else {
cell.thumbnailImageView.image = [UIImage imageNamed:@"rest.png"];
}