Hmm… Tried to run optipng.exe manually in Command prompt window.
When converting PNG->PNG, the program always keeps the original resolution, no matter -preserve option present or not, and rewriting the same or writing to the new file.
When converting BMP->PNG, the program never keeps the original resolution, no matter -preserve option present etc. And with TIF-file it returns an error: "Unsupported compression in TIFF file".
However, when converting any file, even PNG, with the Paint.NET's OptiPNG plugin, in never keeps the resolution.
The only workaround I see now for myself, is to save the file from Paint.NET with its standard PNG-converter, and then compress it by calling optipng.exe manually (e.g. by Windows context menu command).