I think you were changing the bytes in the same way as what I did with Pokémon Black Version 2. Diamond is different from Black 2, try saving the image and put an 8x8 squares on the image using an image editing program like Paint.NET on a new layer and put "0" "1" "2" "3" ect. and once you reach "F", then go back to "0" again. This is so that you'd memorize which squares are which when tile mapping in hex. Each byte goes from left to right on each row and top to bottom for every next row once it reaches the end of the row.
I was editing the Diamond Title Screen with your new tutorial but once I imported it I could not get it to save. It asked me if I wanted to and I clicked yes, but I tried to open it again to test but it was the same old Diamond one. I'll keep trying, but if you try and it works for you then I was doing something wrong. (though I thought that I followed all of the steps correctly)
Edit: I got it to save but I got this: http://img225.imageshack.us/img225/7594/ttdemov1116855.png
Now I'll try the other part! :D