Hi, Matt here and a long time developer.
I'm asking about your project to see if I can help you because I'm confused about what you need.
Steganography is hiding (text) messages into digital images in your case so saving the hidden message to an image cannot be done to the lossy image format (JPG is by default an image compression format that loses details of the original image when saved - this contradicts the steganography that requires lossless compression format unless you had the lossless variation of a JPG format in mind). So images can be saved to PNG or (lossless) TIFF or even BMP and GIF (palette based index of 256 colors).
The second surprise was about your 4 different techniques - first I thought you are talking about steganography methods yet you have stated image saving methods e.g. dither colors to their best approximation (that will produce noise e.g. added pixels in the image - this has to be done before adding a hidden message with steganography and later saving as I understand).
Please correct me if I'm wrong. I'm not into C# much though I know C and could write this program, especially if you point me to links for your techniques. Since I'm programming almost entirely on Mac (so rarely C#) and because of my IT Manager role in the coming EuroBasket Championship U18 Women in my town I've set a longer milestone . If it's possible to implement a software I can and will develop it.
You stated PHP? You need C#, right?
Looking forward to your reply,
Matt