If you don't want to install a (bunch of) new software, you also can download the portable edition of ImageMagick. The current portable version is ImageMagick-7.1.1-43-portable-Q16-HDRI-x64.zip.
The official download page for other versions and other operating systems than Windows can be found here.
After downloading and extracting the ZIP-Archiv, you get 22 files in one directory.
No Setup, no Install, no modification on your system at all, just a plain directory with some command line tools.
The command to modify all your existing PNG and JPG pictures (for example) in a specific source folder into pictures with 200 pixel WIDTH (keeping the aspect ratio) is
mogrify.exe -resize 200x "C:\Temp\IM-Test\T1\*.jpg" "C:\Temp\IM-Test\T1\*.png"
The most easy way to use mogrify.exe is to call it as new Tool and a Windows batch CMD file. Just pass the foldername (for example) from Mp3tag.
mogrify.exe always overwrite the source file!
Details and more examples
The command to modify all your existing PNG and JPG pictures into pictures with 200 pixel HEIGHT (keeping the aspect ratio) is
mogrify.exe -resize x200 "C:\Temp\IM-Test\T1\*.jpg" "C:\Temp\IM-Test\T1\*.png"
If you really want to modify your pictures to a fixed size with a WIDTH and HEIGHT of 200x200 (ignoring the aspect ratio, distorting your pictures!) then you can force mogrify.exe to do this using
mogrify.exe -resize 200x200! "C:\Temp\IM-Test\T1\*.jpg" "C:\Temp\IM-Test\T1\*.png"
There is an incredible amount of additional command line parameters. You can find some examples for -resize here.
Please note that the above examples modify your given pictures directly and without creating a copy in another folder. If such a copy would be necessary, some more commandline options would be necessary. Or you execute a COPY command in your CMD batch file before you mogrify the pictures.
Note (mostly for myself):
To leave the original picture untouched and create a copy with the resized file in an existing output folder with the individual suffix (width x height) you can use magick.exe with
magick.exe "C:\Temp\IM Test Dir\T1 - T\*.jpg" -resize 200x -set filename:f "C:/Temp/Existing Output folder for mogrified pictures/%t (%wx%h)" "%[filename:f].jpg"
Don't forget to use double %% characters inside a CMD batch as
magick.exe "C:\Temp\IM Test Dir\T1 - T\*.jpg" -resize 200x -set filename:f "C:/Temp/Existing Output folder for mogrified pictures/%%t (%%wx%%h)" "%%[filename:f].jpg"
This would create a copy of "My example picture.jpg" (1076x911 Pixels) from the given source folder, resize it to a width of 200 pixels and write it in the given output folder as "My example picture (200x169).jpg". The source picture remains untouched.