just take a black magic marker and draw a line across the bottom of your screen then you wont see the white line... 
seriously though, I know nothing about LS but I do know a little bit about SKS and bmp's. is there any transparent parts in your image? are you sure the "magic pink" is true magic pink (255 - 0 - 255), if using a layer based software like photoshop did you flatten the image? if so don't, that will change the background from magic pink to white.
in other thoughts. will LS accept .png files? I don't know but I find png easier to work with than bmps because transparency in pngs is simple as a tranparent background not a "magic" color.