Mapping photoshop images to touchscreen coordinates

I am wondering if someone could help me understand how people create rich iphone like graphical interfaces using cheap tft touchscreens and photoshop.

I want to achieve something like this

Where each button performs an action and the page is able to update stats such as temperature etc.

From my understanding the person
1.Drew these images in photoshop
2.Loaded them onto an sdcard
3.Used UTFT_tinyFAT to read the graphics from the sd card to the display.

Now this is where I become confused. How can I map the pixels of the image to touch coordinates in an easy way? How would the pros do this?

The way I can think of doing this is printing an overlay and manually sticking it onto the shield to figure out where each button is and then coding it and its relative actions into my code. Or by good old trial and error!