Can anyone please explain to me how to compute DCT? I'm finding it hard to understand it. 2D-DCTdct1 I attached the formulas the Discrete Cosine Transform (DCT) and it's Inverse-DCT.

I read something about compression. Should I compressed the image after i watermarked it. I'm basically goint to use DCT for image watermarking.

I'm assuming you mean a visible stamp on the image. Yes, you should compress AFTER watermarking.

No it's an invisible watermark

by which method we can embedded the original image with watermark image?