关面说到uiwebview不能识别long press gesture，幸好有一个可以识别，那就是double click.因此我们注册它，代码如下：
UITapGestureRecognizer *doubleTap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(doubleTap:)]; 
doubleTap.numberOfTouchesRequired = 2; 
[self.theWebView addGestureRecognizer:doubleTap];
然后就是实现doubleTap:
-(void) doubleTap :(UITapGestureRecognizer*) sender 
{ 
// <Find HTML tag which was clicked by user> 
// <If tag is IMG, then get image URL and start saving> 
int scrollPositionY = [[self.theWebView stringByEvaluatingJavaScriptFromString:@"window.pageYOffset"] intValue]; 
int scrollPositionX = [[self.theWebView stringByEvaluatingJavaScriptFromString:@"window.pageXOffset"] intValue];

int displayWidth = [[self.theWebView stringByEvaluatingJavaScriptFromString:@"window.outerWidth"] intValue]; 
CGFloat scale = theWebView.frame.size.width / displayWidth;

CGPoint pt = [sender locationInView:self.theWebView]; 
pt.x *= scale; 
pt.y *= scale; 
pt.x += scrollPositionX; 
pt.y += scrollPositionY;

NSString *js = [NSString stringWithFormat:@"document.elementFromPoint(%f, %f).tagName", pt.x, pt.y]; 
NSString * tagName = [self.theWebView stringByEvaluatingJavaScriptFromString:js]; 
if ([tagName isEqualToString:@"img"]) { 
NSString *imgURL = [NSString stringWithFormat:@"document.elementFromPoint(%f, %f).src", pt.x, pt.y]; 
NSString *urlToSave = [self.theWebView stringByEvaluatingJavaScriptFromString:imgURL]; 
NSLog(@"image url=%@", urlToSave); 
} 
}
这样我们就可以得到图片的url,然后下载保存就行了。