Fourier

I'd like to understand
Fourier in image transform

I have learnt in my school that
F(x,y)=f(x,y)=C*(long summation)

Given an image 16x16, and its pixel data after read are stored in imgData of (BYTE*imgData)

What is x,y and how can I apply F for imgData ?

Thank you

Sounds like you are trying to compare images, right,
I read somewhere people state that there are many methods usable to do this and none of them offer 100% correctness. I used to make a similar question to compare two images but no one answered me. It's odd that many software, tools still available out there and can offer detection of xx% similarity between images in incredible speed even when searching the whole drive. And those are for free. It's selfish that they know clearly a very easy method, seems so, to do this but they keep always silent!
All in All, I need too a simple approach to find xx% similarity between two given images :( Someone who can help is highly appreciated.