PDN v3.5.5 (Final Release build 3.55.3767.28608)
Windows 7 Ultimate 64-bit with .NET Framework 3.5SP1 & all latest updates as of 4/28/2010
This is a dev machine with VS2008SP1 & Expression 3 installed.
I opened a JPG, cropped it slightly, and then saved it as a PNG. I chose the PNG file type in the "Save as type" combo box. In the Save Configuration window, the preview shows only a small black dot centered in the window and the label "Preview, file size: (computing)" never changes from "(computing)".
Bit Depths of Auto-detect, 32-bit, and 24-bit produce the same results. Selecting a bit deptch of 8-bit causes the file size label to toggle continuously between "(computing)" and "(computing: 100%)".
The resulting PNG file cannot be displayed in Windows Photo Viewer or opened with PDN. I have attached a sample of the output file generated as above with the explicit selection of 24-bit PNG.
Save As JPG and BMP work fine. Pasting a screen capture into a blank image and saving that as PNG works, too, so it may be something specific to this image or the fact that the source image is JPG.
I can upload the source JPG image if desired (it is 644K).