picmess

Hide a message in the pixels of an image (LSB steganography) - runs entirely in your browser, no server needed.

Image source
Image with a hidden message

The message is encrypted (AES-256-CTR + HMAC-SHA256) and hidden in the least significant bit of the R/G/B channels of successive pixels - a color change of at most 1/255 per channel, invisible to the naked eye. The result is always saved as PNG (a lossless format - JPEG would destroy the hidden bits). The key is also appended in plain text after the file's IEND marker - a second, independent channel for hiding data that browsers and image programs ignore.

Note: fetching images by link only works if the remote server allows CORS - if the link fails, download the image to disk and upload it as a file instead.