Using WebThumb API, you can generate a thumbnail in three steps. First you have to place a request containing the URL. As soon as your request is successful, WebThumb store your request in queue. That means you are not getting the thumbnail instantly (well, there are other factors also. to fetch an url requires time, so it is not possible to generate the thumbnail in real time).
In second step you have to check whether your thumbnail has been generated or it is still in the queue. If you get a green signal, you will proceed to the third step where you have to request a download URL of your thumbnails.
He shows how to make a request to the API, check how your request is doing (status), and grabbing the thumbnail it's generated. Then, it's on to the code, and a complete PHP script (using cURL) to make the complete request to the API, including waiting for the image to be finished to grab it.