Can you do this from within Word using any settings? If no, than you won't be able to do it using automation either. If yes, do a macro recording, and see, how the built-in automation is performing it. Since that's also VB, it will be easy to transcribe it to VB.NET.
] will allow you to save as PNG, but as I know, you can't infliuence downsamoling (see: http://answers.microsoft.com/en-us/office/forum/office_2010-word/image-quality-changes-when-saving-as-web-page/4c860944-c627-4fed-afd4-a7473ab1dee6
You could make tough a two-pass export: use non-filtered export to get the high-resolution images, and the filtered one to get the rest.