第一步：

- (void)webViewDidFinishLoad:(UIWebView *)webView {
    NSString * imageJavaScript =
                  @"function assignImageClickAction() {                                 \
                        var a = document.getElementsByTagName('img');                    \
                        var b = a.length;                                                \
                        for (var i = 0; i < b; i++) {                                    \
                            img = a[i];                                                  \
                            img.onclick = function() {                                   \
                                window.location.href = 'image-preview:' + this.src       \
                            }                                                            \
                        }                                                                \
                    }";
   
    [self.webView stringByEvaluatingJavaScriptFromString:imageJavaScript];

    [self.webView stringByEvaluatingJavaScriptFromString:@"assignImageClickAction();"];
}

第二步：

- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType {
    //預覽圖片
    if ([request.URL.scheme isEqualToString:@"image-preview"]) {
        NSString* path = [request.URL.absoluteString substringFromIndex:[@"image-preview:" length]];
        path = [path stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];

        [self.imageView setImageWithURL:[NSURL URLWithString:path] placeholderImage:[UIImage imageNamed:@"default"] usingActivityIndicatorStyle:UIActivityIndicatorViewStyleWhite];
        
        [UIView animateWithDuration:0.2f animations:^{
            self.imageView.alpha = 1.0f;
        }];
        return NO;
    }
    return YES;
}

*****關鍵點*****

            image-preview    這個名字兩個webview方法里得一致