After installing CyanogenMod 7 on my HTC Desire, I wanted to have a boot splash (the image that appears before the boot animation) to match. I already knew well how to change the boot splash image on HTC Desire, I just needed a nice picture.
For the impatient ones, the image that I made looks like this:
You can download it in NB format and flash it with “fastboot flash splash1 cm7.bmp.nb” with your phone in fastboot mode.
If you want to know how I made, so you can make your own cool boot splashes (for example, to write your name and a phone number, in case you lose your phone), read on.
Step 1. I extracted to stock CM7 animation from the system image. I simply opened the file update-cm-7.0.0-Desire-signed.zip I downloaded (7.0.2 is the latest version, you can use that one) and extracted /system/media/bootanimation.zip. This is the boot animation.
Step 2. I unpacked the file bootanimation.zip. What I got was a bunch of png files – the individual frames of the animation.
Step 3. I picked one that I like – /part1/CM7_256_073.png. As the file is a 256×256 PNG, and I needed a 480×800 BMP, I had to flex my Gimp muscle (Nicu would be proud of me!).
Step 4. Fired up Gimp and resized the image to our target width. I used the option Image > Scale Image > 480×480 with Sinc Interpolation (I think the image looks less grainy with this one).
Step 5. I then increased the height of the image with Image > Canvas size. I needed to click the chain to unlink the X and Y sizes, so I could enter 480×800. Then I pressed Center and I choose the option to resize all layers.
Step 6. Fill the empty parts with black. I selected the Bucket Fill Tool and the black color, then I clicked on the top and bottom parts of the picture. Now my image looked like I wanted it.
Step 7. Change to RGB mode and export as BMP. This is done with Image > Mode > RBG, followed by File > Export and writing the file name “cm7.bmp” (Gimp knows what format to save based on extension).
From now on, the steps are exactly the same from my original post on changing the boot splash image.
Step 8. Check the BMP file is what nbimg expects:
$ file cm7.bmp
cm7.bmp: PC bitmap, Windows 3.x format, 480 x 800 x 24
Step 9. Convert to NB format:
$ nbimg -F cm7.bmp
After conversion, the file size of cm7.bmp.nb must be exactly 768032 bytes!
Step 10. Reboot the phone in fastboot mode and flash the image:
$ fastboot flash splash1 cm7.bmp.nb
Step 11. Reboot phone and enjoy! 
Download links:
Image credit: laihiuyeung ryanne.
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