I correctly find the URL encoding the whole href, confirming the ampersand (&) encoding as inside the href attribute. In addition, it encodes as% 26 inside the url parameter and moves the url parameter to the last place in the href attribute, correctly creates a link to the page containing the image when you click on your PIN code.
Example:
<a href="//pinterest.com/pin/create/button/?media=http%3a%2f%2example.com%2fImages%2fimage.jpg&description=Example+description.&url=http%3A%2F%2Fexample.com%2FExample%2FExample.php%3Fparam1%3Dvalue1%26param2%3Dvalue2%26param3%3Dvalue3"><img src="//assets.pinterest.com/images/pidgets/pin_it_button.png"></a>
eniac source share