Sounds like you are wanting to display a HTML page which displays something like an image that changes in height as the value of your ultrasonic changes, is my understanding correct ?
Also, you are wanting to do this without using any other devices, such as servers on a hosted site, just using the Arduino and router to connect it to the internet, is this correct ?
If this is correct, how about this for an idea.
Firstly, forget about using php, you will need a server that has php, and the Arduino doesn't, can't support that.
Next, instead of using an image would it be a better idea to use HTML5 and use the 'canvas' element and then use HTML stroke and fill commands to draw your shape, say a rectangle. The rectangle would be sized according to your value from your ultrasonic sensor.
I imagine this would be quite a small and simple html document.
You could also look at using jQuery and have the client side request the data from the Arduino typically in json format every few seconds as you wish.
Using this concept is very neat and allows you to expand upon your idea as your program starts to grow.
Have a look at something I am working on in my spare time.
I have an Arduino Mega with on board ethernet and SD card connected to my router.
The client side request data directly from the Arduino using json format.
Also, the Arduino pushes data up to the host server where I have a php file take that and store in a SQL database.
It is still a work in progress at this point in time.
But it might give you some ideas of what you can do.
Go to http://paulalting.com/hydrosolar.php
Regards,
Paul