various devo 12s incomplete wallapers

Hello and sorry by my horrible english.

I make some devo 12 wallapers but in 24 bits, i don´t know how convert .bmp images to 16 bits to devo 12.

Please, can you convert this bmp images to 24 at 16 bits? THANKS very much!!!!!

These are wallapers:

I use GIMP for making icons and wallpapers. It allows you to save in the correct format.