I realize that this thread is almost a month old, but it's still on the first page, so hopefully that's considered recent enough
I've got two solutions that would both help towards making the encoded image perceptually invisible
For example, say you want to encode a 1-bit image into a 24-bit image. You're commandeering, I imagine, the least significant 3 bits (one per channel) in each pixel.
First, for a 1-bit source image, you should get three hidden pixels per one real pixel. If we're only encoding a 1-bit image, might as well take advantage of all the available bits. This also means that the hidden image can be up to three times larger (in number of pixels) than the final image.
The second suggestion, and this is the most effective part, is to apply a compression algorithm to the image you're hiding (.NET directly supports several compression systems already). Compressed files look a lot like random bits. This is because a good compression algorithm produces data that has few patterns. Because patterns are compressible, that'd mean that the compression algorithm wasn't doing a good job.
Just to sum that up, when you compress data, the output looks random. So if you take a 1-bit image, compress it using something fairly simple (deflate, for example), and THEN overlay the compressed data (one bit at a time) over the final image, the overlay will look random. There won't be any pattern, and slight random variations in pixels are pretty much invisible to the human eye!
The other benefit, of course, is that since we're taking a 1-bit image, encoding 3 bits per pixel, and compressing it, you can either fit significantly larger images in, or work in redundancy to survive lossy compression. Or you could just repeat the data to get a uniform random jitter on all pixels.
EDIT: Here's an image with a 1-bit random image (OK, semi-random, it was the PDN Cloud plugin with minimum size, and then a threshold applied, it looks random enough). it gets added as just slight noise being added to the photo, no chance to see it on top of a photograph:
Except that slight random noise could be holding an actual image once you extract and decompress it.