Is there any docx to image (jpg, png, bmp, ...) available with
command line support? It's better to have one in windows, but if there is any in linux, that's great too.
VeryDOC's DOC to Any Converter claims to be able to do what you want. However it's not free - 79$ per End User License of the CLI version.
It's as easy as:
c:\doc2any_cmd>doc2any.exe mydocument.docx image_of_mydocument.bmp
You can download a trial version to see if it's upto your expectations. But it will riddle the output files with fugly watermarks.
I realise this is a very old question, but it comes up high on search results so warranted a response. As another option that is not a paid product, take a look at libreoffice.
You can use their command line interface to convert documents that it can open to pdf. From there, you can use imagemagick to convert pdf to image.
A very basic example of how you can use libreoffice can be found in this discussion on askubuntu: https://askubuntu.com/questions/396825/convert-docx-to-pdf
Converting then from pdf to image would be something along these lines: