This article shows 3 ways to convert a color InDesign document to grayscale (black & white) - Export PDF, Use color blend or Export image.

Windows does not have a native tool to convert your color .PNG image files to grayscale, the technical term for "Black and White." If you have a .PNG that you want to convert to grayscale, you do ...

By default, images added will open in RGB mode, but the option is there to change it to grayscale, so let us talk a little bit about how to accomplish that. Open the GIMP app. From there, press ...

Select all the images you wish to convert, right-click on the selection, and select Quick Actions > Convert to JPG. If you named your quick action something else, choose it from the Quick Action menu.

The image file converter on my MacBook Pro is awesome for doing all of the above quickly. It only lets you convert to JPEG, PNG and HEIF. However, it will allow you to convert from other file formats.