nope
nothingburger
a pixel is just a pixel, it contains a value for red, green and blue, each can vary between 0 and 255. that is enough to encode around 3 letters, but without the context of other pixels it is just a color.
encoded messages in images is called steganography, usually done by carefully manipulating the differences between pixels. here is a good example: https://en.wikipedia.org/wiki/Steganography
true, and in practice JPG compression means there is slightly less than 3 bytes' worth of data per pixel. just trying to keep things simple though.
I suppose it could, but this would be a pretty dumb way to encode information. a better example is a barcode or QR code, it's data in an image that can't be read by the naked eye but can be decoded by systems designed for the purpose.
that kind of hidden message is usually more in symbolism and words, remember when Q decoded the Maggie Haberman tweets about the Q train, smoking, etc?
sure, as other anon says, theoretically possible, but depends on the systems involved (resizing, further compression etc) and the encoding method used (both parties need to agree on this ahead of time)