With any image, how much change in a pixel can happen, before you notice a change? For a computer, even if only one bit in a thousand changes, it can notice... as long as it has the original image to compare against. But for stenography, you could use a picture you just took and destroy the original as soon as you used it to hide the data.

For a human... a 24 bit image has a possible about 16M colors. The human eye is said to be able to distinguish 10M colors. If you take a bit out of every byte, at the least significant bit position, or 3 bits out of every 24 bit pixel, you reduce the color availability, down to just north of 2M colors (2^21). You can see that in the image below. It shows a background with rectangles where I have modified 1, 2, 3, 4, and 5 bits. Anything more than 1 or 2 bits and it gets noticeable. Take into account that this is larger swatches, noticing discrepancies at the pixel level would be harder for the human eye. And also take into account that here you have a base comparison, the background. In a pixel by pixel change, and without the original image, there would not be that clear baseline for the eye to catch on. A change of 1 or 2 bits, in the least significant bits, would be hard to detect.

But let's play it safe and only take one out of every 8 bits, and being smart, we take the least significant bit out of every color in every pixel. That means we have one eight of the original 786432 bytes available to us - 98k. Compress the message you want to hide in the picture, then encrypt it (compress first!). Stick it in that 98k. You are golden.

Short answer... 98k compressed, easy, and forget detection... compression alone randomizes the data you are sending; encryption further randomizes your compressed data. Random bits in the least significant color bits of each pixel... how can you tell from noise?

With any image, how much change in a pixel can happen, before you notice a change? For a computer, even if only one bit in a thousand changes, it can notice... as long as it has the original image to compare against. But for stenography, you could use a picture you just took and destroy the original as soon as you used it to hide the data.

For a human... a 24 bit image has a possible about 16M colors. The human eye is said to be able to distinguish 10M colors. If you take a bit out of every byte, at the least significant bit position, or 3 bits out of every 24 bit pixel, you reduce the color availability, down to just north of 2M colors (2^21). You can see that in the image below. It shows a background with rectangles where I have modified 1, 2, 3, 4, and 5 bits. Anything more than 1 or 2 bits and it gets noticeable. Take into account that this is larger swatches, noticing discrepancies at the pixel level would be harder for the human eye. And also take into account that here you have a base comparison, the background. In a pixel by pixel change, and without the original image, there would not be that clear baseline for the eye to catch on. A change of 1 or 2 bits, in the least significant bits, would be hard to detect.

But let's play it safe and only take one out of every 8 bits, and being smart, we take the least significant bit out of every color in every pixel. That means we have one eight of the original 786432(pixels) * 3 bytes (24 bit per pixel) available to us = 294k. Compress the message you want to hide in the picture, then encrypt it (compress first!). Stick it in that 294k. You are golden.

Short answer... 294k compressed, easy, and forget detection... compression alone randomizes the data you are sending; encryption further randomizes your compressed data. Random bits in the least significant color bits of each pixel... how can you tell from noise?