what format and how exactly make a .blp file

[hakowy]

hi, an about year ago i had to make this .blp file in wc3 viewer II http://www.savefile.com/files/1411962.
When i vieving it in wc3 viewer its looks like .gif with transparency background. This way i want make new .blp files like this, but i don't know/forgot what exactly size,format,colors must have a .blp file to work and shows properly in wc3 map.
I tried lastely to convert in wc3 viewer from bmp,tga but i couldn't do that..