Is it possible to write something into a pixel?

No not really.

A typical JPEG encoded image stores 24 bits of data per pixel (this data is used to describe the colour of the pixel). A single character in most character encoding schemes take up 8 bits. So you could potentially store 3 characters in a pixel. That would be a pretty short secret message. Plus the pixel would most likely not be the correct colour any more (since the pixel data is no longer describing the colour).

What you can do, is hide a secret message in an image, this is called image steganography.

You can try that on this site.

http://mozaiq.org/encrypt/

If you are thinking of something like a microdot the answer is no. Code can be written that will trigger off when a pixel has focus but code can NOT be placed in a pixel itself. The old Minesweeper that came with windows had code behind it that would change the pixel in the top left corner to a different color when you were over a bomb but you had to have really good eyes to see it

The smallest a screen can generate anything is 1 pixel. So 1 pixel can at most contain a single color.