Fix BMP loading in DCT sample

This fixes the BMP loading routine in the DCT example, which expected a hardcoded set of headers and got it wrong for the example image. Instead, use the header information to jump to the image data. Also updates the set of pre-processed images. Before: <img width="512" height="512" alt="image" src="https://github.com/user-attachments/assets/7a8580e7-fef2-41a5-876a-7899e49de22f" /> After: <img width="512" height="512" alt="image" src="https://github.com/user-attachments/assets/f6cf8b7a-41a0-466e-97b1-d4a66d0b83e2" />