Question about DCT

i have a question about DCT..actually it's sound like silly question but it's important for me..
The question is:
1. Why on DCT the function use a cosine function, if we see on the image and the
function there is no connection with angle.
2. Why the function using cosine, why not using sine or tangen? what is the influence
for the compression ratio?.
3. Is it true that we using the pixel depth for making 8x8 matix as the input for DCT,
if it's true how to get a pixel depth on image in C# so we could make the 8x8 matrix.

thanks.. God bless you