Just to be sure, you're using 6.1.13.1? Find this by selecting the Help>About Bryce menu item while holding down the Control (PC)/Command (Mac). Here's a test I just did with PNG (also did TIFF). Left is the original, right is the image import. I used the down arrow pop-up at the top of the Terrain Canvas window and selected the Export Image item to export the image. I'm also at home with only a Mac, so this may be a PC issue.