First, don't bother and save from Photoshop with transparency. Second, imagemagick is the key background to set the background color. Third, it is unclear why save the black-and-white images in the JPEG to PNG without transparency they will not be a lot to take.
Herbert answered on March 23rd 20 at 18:54
PNG with transparency, most likely. The alpha channel in different programs is interpreted in different ways. Maybe black, maybe white, maybe as checkered. The conversion goes like this: PNG->preflight (something like a BMP in memory) with the information layers -> compression to JPG. Here the alpha is filled with the colour of the background substrate, in this case black. Black text on a black background!
To check in word make part of the text, for example, red. Posmotrite what happens