Where the message is
Every pixel is three numbers: how much red, how much green, how much blue. On this site those numbers are written in hexadecimal.
Change a blue value from 228 to 229 and no eye on earth can tell. So use that last digit: even for a nought, odd for a one. Eight pixels carry one letter.
Press the three buttons in order. The picture, then only the blue channel, then only that last digit drawn as black or white. The message was in the picture the whole time.
The least significant bit
That last digit has a name: the least significant bit, the part of a number that matters least. A photograph has millions of them going spare, so a picture of a cat can carry a book.
It is the oldest trick on the internet and the direct descendant of everything else in this section. The microdot made the message too small to notice. This makes it too quiet to notice.
And it is fragile in a way ink is not. Save the picture in a format that squeezes it smaller and those last digits get rounded away. The image still looks identical, and the message is gone. Hiding in the quietest part of something means anyone who tidies up destroys your letter without ever knowing it was there.
Back to HIPS The microdot HexadecimalRead more elsewhere
Longer, harder and written for grown-ups. Worth a look when this page runs out.