I think the renaming to .doc is actually what messes things up. Though Word can open things fine like that, when saving it doesn't format it exactly how it looks... thus corruption occurs if you try to use this method for hex editing. Your best bet is to find a good hex editor (Google up "XVI32" for a winner) and use it to add the (C)Pixel.
Alternatively, to save yourself a *huge* amount of trouble, use the aforementioned hex editor, open your Doukutsu.exe (backup first!), and search for all instances of (C)Pixel in the .exe. For each character of (C)Pixel, change the corresponding hex numbers (left side) each to 00. It'll look like a bunch of boxes where (C)Pixel used to be, but after saving, you won't ever have to worry about hexing your image files again. Works like a charm for me. ;P