Hi Greg,
I was able to find the problem source. In the ScriptLibrary "ImportPicture" we have a function called "ReadJPGFileInfo" which returns the resolution of a give file. If this routine fails to detect the image size due to an unknown image format it returns 0. So the image is displayed on your website with 0x0 which means invisible.
As a workaround net next release 0.9g will check for 0 size resolution information. Negative impact is just not having fullsize images and maybe no shadows.
To fix your already imported pictures open the view "All Pictures", select all affected documents and run both "Update pic's .." agents from the Actions menue.
Chris