Can't convert to .Blp

hey, i don't know if this question is answered in the forums already but i couldn't find it, anyway i've been trying to convert some stuff to blp with Warcraft 3 Viewer but i can't get it to work. I have tried having it as a .tga with an alpha channel (I use Paint Shop Pro 7). and then i tried to use a bmp, so that the viewer automatically gives it an opaque alpha channel. Everytime I try to convert something it says that it worked fine, but the resulting blp is 0 bytes and doesn't work. I want to be able to make a screenshot a blp so that i can make a loading screen.

i think u mean a map preview actually, and in that case ,.... well in both cases, im afraid only TARGA files will do the trick... actually. ive been playing around with map preview screens for the last around 7 hours of my life and mine still dosent work.. i found a AWESOME pic and did it up for my map preview (the seige of Isareal) and prepped it and it diddent work! i made a new file ( i used photoshop 5.0) that was 256X256 24 bit and pasted the pic in, then saved! YES i thought! finally got it! Yes.. that saved as a Targa, but then i added the alpha channel and it would only save as a bitmap or gif etc !
im confused,,, and if u do amnage to work it out tell me.. im playon around now and will post any results here! :<