I've always make uis2 frames in png, this time i've try in bmp but the result is not correct......i've put a layer in background with "magic pink" (255,0,255) but on the rounded edges the pink don't disappear....where is the error?

I've made only right and left borders....the ground grey in the picture don't help but cursorfx is the only program that i have on pc that use uis2 
Please help