The example Use steganography to hide one picture inside another in C# explains how you can hide one image inside another. This example uses the same tools to hide a large image inside another large image so you can get a better view of the results. Switch back and forth between the images to see how much they changed when you store each image with a different number of pixels.
The images used here are 600 x 700 pixels = 420,000 pixels in total, so they contain 3 * 420,000 = 1,260,000 color component values. If you use 4 bits to store hidden information, which seems to be about the most you can reasonably get away with without noticeably altering the visible image, then you can store 4 * 1,260,000 bits or 630,000 bytes of hidden information.