Hide a message in a picture
Every pixel's colour is three numbers from 0 to 255. Change the last bit of each one and the picture looks exactly the same, but those bits can spell out a secret. Only someone who knows where to look can read it.
Now hide your own
Use the sample photo or one of yours. Everything happens on this device: your photo and your message are never uploaded.
The bottom two show the top-left corner of the picture, enlarged, because the message is written from there, pixel by pixel.
How it works
Hiding data inside something ordinary is called steganography, from the Greek for "covered writing".
Each pixel mixes red, green and blue, each from 0 to 255. A photo from your phone has millions of these numbers.
Turning a red of 200 into 201 is a change of less than half a percent. No eye can see it, so that last bit is free to carry something else.
Text is stored as bytes of 8 bits. Spread them over the last bits of the pixels, one by one, and the picture now carries your words. An Arabic letter takes two bytes.
Hidden is not the same as safe. With a password, the text is encrypted (AES) before it is hidden, so even someone who finds it can't read it.
The other side of hidden pictures
This trick is fun for puzzles and secret notes. It is also used by people with worse plans, so it helps to know how it works.
Some harmful programs download innocent-looking images that secretly carry their next instructions or code. A picture can't run anything by itself: the danger is a bad program already on the device. Install apps only from official stores and keep your devices updated.
Someone could smuggle files out of a company inside ordinary-looking photos. That is one reason some workplaces check or limit what can be uploaded and sent out.
Send a picture as a normal photo and WhatsApp and most chat apps shrink and re-compress it to save data. That compression is lossy: it rewrites the tiny details, so a message hidden in the last bits is destroyed. Sent as a document, the file arrives unchanged.
This simple method fools the eye, not the expert. Forensic tools can spot the statistical traces that hidden data leaves. A password keeps the words unreadable, but not the fact that something is there.
Sources: MITRE ATT&CK, the public knowledge base of attacker techniques, lists steganography as a technique used by real malware to hide code and commands (T1027.003) and hidden data (T1001.002). JPEG compression is lossy by design (ITU-T T.81 / ISO/IEC 10918-1); the "send as a photo" test on this page runs a real JPEG re-compression on your device so you can see the effect for yourself. Word counts are estimated from the average length of words on this page in your language; a book page is taken as roughly 300 words.