At Tools for module developers you will find a list of tools, with links, that are useful for module development.
For example, the tool ImageMagick is really great for doing batch (multi-file) operations on multiple images. To install it, open a terminal (Win+R - or 
+R and then write cmd in the dialog and press OK). This will open a new window, which is entirely black with white text on it. You will see a command prompt
C:\Users\username>
where username is your user name.
In that terminal type
C:\Users\username> winget install ImageMagick.Q16-HDRI
and press enter. That will install the application.
Now, if you have unpacked the images into a directory - say C:\Users\username\Document\D-Day\images, then you can change to that directory
C:\Users\username> cd Documents\D-Day\images
To list the files in that directory, do
C:\Users\username\Documents\D-Day\images> dir
Create a new sub-directory for the converted images
C:\Users\username\Documents\D-Day\images> mkdir new
You can now run magick on all these files with
C:\Users\username\Documents\D-Day\images> mogrify -format png -path new *.png
or
C:\Users\username\Documents\D-Day\images> for %f in (*.png) do magick "%f" "new\%f"
The converted images are now all in the sub-directory new, and if you are happy with them, you can move them to the images directory.
C:\Users\username\Documents\D-Day\images> move /Y new\*.png .
C:\Users\username\Documents\D-Day\images> rmdir new
With the above magick, you will probably not gain much - if anything - from applying optipng to the images also.
Yours,
Christian