I put together a little tool for editing the bitmaps in Walkera Devo 6, 8 & 12 after chatting to FDR_. You can find it here
When you open a Walkera lib you will be presented with a list of "resource elements" each of which contains one or more bitmaps. Double clicking on a "resource element" allows you browse the bitmaps in that element and import/export the bitmaps as standard Windows bitmap files. When importing a bitmap it must be the same size as the original.
You can also export all the bitmaps in a Walkera lib file (use the export all button).
If you modify any of the bitmaps in the file you can update it with your changes and the app will fix up the CRC for you. The modified file can then be flashed to your Tx as usual - obviously use a lib file that matches your Tx's firmware
Note: This app is provided "as-is" and with no warranty. If it bricks your Tx then I am not responsible. That said, it really shouldn't since all we are doing is changing the contents of the bitmaps.
Edit: Update v0.2 tool can be found here
Adds support for import/export of strings. Strings will be extracted to a .rtf file
European languages, make sure your editor handles UTF-7
Chinese, make sure your editor handles Bigendian Unicode
For some reason the Chinese strings don't display correctly.
Edit: Update v0.2A tool can be found here