At the asda photo booth it says you can print photos via Bluetooth, read on?

I have an iPhone, how do you do it from iPhone, like what buttons do you press?

Unfortunately, iPhones have extraordinarily bad support for Bluetooth. You can use it to connect to a Bluetooth headset, but you can't transfer pictures via Bluetooth. It sucks.

The way I do it, I copy the photos from my iPhone onto my computer with iTunes, and then copy those onto a regular USB memory stick. The machines always read the memory stick no problems.