Hello again
A new day and new ideas. Preview problem is solved !
Here my findings:
1) The size of the preview needs to be square e.g. 512x512px and NOT 1024x768 as mentioned in the downloadable PDF description about how to repaint !!!
2) The graphic conversion tool which creates ttx (af5-makemaps.exe) expects a special png file named preview.png. If this is not present an error is printed through conversion (but the rest is still done). It seems this tool doesn't handle transparency info in a PNG as expected.
The work-around and clue is to create a Tiff with transparency info and convert it like a normal texture file, as described by Jet-Pack.
So by converting my PNG with transparency (preview_color.png) into a Tiff with transparency (preview_color.tif) and then running the "af5-makemaps.exe" to get a preview_color.ttx and finally renaming it to preview.ttx solved my problem.
A long way, but a solution in the end.
P.S.: Will keep you informed once the repaint is uploaded
Thanks
Mike-DE