Image editing , C# //not C# specific

[totonex]

Hello, i'm doing something in C#...that's why i'm poting it here.
I will make an application which hides text in an image, by changing the image's least significant bits, to reproduce the ASCII equivalent (A is 65, which is 0100001) of each of the text's letters.
Problem is, i can't just open an image in a binary format, and carelessly sifting through the byte array. For example, BMP's have a 54-byte header, so the algorithm should start chopping bytes only from the 55th byte.
Modifying the header would destroy the image.
My main question is:
Does anyone here know precisely this kind of information for other formats (GIF,JPG,PNG) ? I've searched quite a lot, and haven't been able to find information about them. I need information regarding header length - i need the byte number at which data about pixels starts. (e.g. in BMP's it starts at the 55th byte) .
And also ...since it is a C# thread. Methods for opening a file binary, and processing it as a byte array.
Thanks.

[lobo521]

In C# there should be some Image class.