I've encountered that many times. Even i have the .bmp still it can't read my GRF.
All you have to do is like this
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1. Extract your .bmp on GRF
SAMPLE KAZEKAGE_HAT.bmp
2. After extracted. Open your KAZEKAGE_HAT.bmp on any picture editor
SAMPLE (COREL DRAW, PHOTOSHOP)any version <-----WORKING
SAMPLE (MICROSOFT PICTURE MANAGER) <----- Not Tried
3. Edit or put anything even small . on the picture to read it as new .BMP of any picture editor.
4. Save it using SAVE AS, then choose .bmp
TIPS: Make sure the width and height should be same as before.
5. Put it again on your GRF at ITEM & COLLECTION FOLDER. then save.
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