Change the name of the file into zip or rar.
For example :
You have a bootskin called AMD.bootskin
Changed its extension into AMD.zip or AMD.rar
Now Unzip or Unrar the file
You will see a bitmap a file called Bootskin.ini and a bitmap called Progress.bmp
The two bitmaps are the screens you'll see when the "bootskin" is running at the boot of Windows.
You can also made a new bootskin. Simply call the one Bootskin.ini and one of your bitmaps Progress.bmp and zip or rar everything.
Than you rename the extension of the zip or rar file back into a bootskin