You should take a look at Steganography. And be aware you are not hidding your data in the image. Anyone who could open the image with a text editor would see your text somewhere in the file (in this case, in the end, which is much worse). If I were you, I'd do the following:
- Encrypt your data with some decent Algorithm and a strong key
- Create a function that distributes your data through the file in a pseudo-random way, so that anyone would note that you're trying to put something secret in it (be aware you have to recover it afterwards). In a regular bitmap image, you can use the last bit of each pixel to save your information, since this change made by it would not be perceived by human eye, if you compared the original image with the one that has hidden data.
- Pray NSA isn't reading this, otherwise you can get some serious trouble :)