I think the idea of visualizing the frequency content of images is interesting. I wonder how this is achieved? How to get the frequency component of a pixel?

It's more about the variation across pixels. It's the same as obtaining the frequency information from a simple sinusoid. One point on the sinusoid won't tell you anything. The more points, the more information. One thing to note is that there is a cap on spatial frequency information introduced by the fact that images are discrete pixels. Specifically, the highest frequency variation we can observe exists when two adjacent pixels are uint8 values 255 and 0 (or vice versa). When this occurs, we have 0.5 cycles/pixel.