Попробуйте что-то подобное в вашем делегате webView:
- (void)webViewDidFinishLoad:(UIWebView *)webView
{
CGSize thumbsize = CGSizeMake(100,100);
UIGraphicsBeginImageContext(thumbsize);
CGContextRef context = UIGraphicsGetCurrentContext();
CGFloat scalingFactor = thumbsize.width/webView.frame.size.width;
CGContextScaleCTM(context, scalingFactor,scalingFactor);
[webView.layer renderInContext: UIGraphicsGetCurrentContext()];
UIImage *thumbImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
//throw it on screen, just for testing
UIImageView *imgView = [[[UIImageView alloc] initWithImage:thumbImage] autorelease];
[self.view addSubview:imgView];
}