Are you using Wc3 Viewer? If memory serves I had that problem too. I think it occured when tried to convert pictures larger that 256x256. I know no way to solve it.
Edited 21 Juni 2005 I tested a picture of size 256x256 a which worked fine. The I tried 257x257 and it didn't work. The picture was all blank. The program can't convert larger pictures than 256. Too bad.