I'd recommend Imagemagick. Something like this should work (untested)

FOR %a in (*.jpg) DO convert %a -resize 600x600 -background black -gravity center -extent 600x600 square_%a

Full steps to get this working when you're unfamiliar with scripting are as follows (written for Windows, but similar steps exists for Ubuntu and Mac OS):

1. Download ImageMagick.
2. Make a copy of the files you want to work on in a new folder (just in case something goes wrong).
3. Open the command prompt through the Windows start menu.
4. Navigate to the folder created in step 2 by typing cd path/to/new/work/folder into the command prompt.
5. List all the files in the folder with the command dir.

6. Try out the ImageMagick command on one file to see if the results are satisfactory. Adjust the file_name0001.jpg part to the filename of the image you want to try the script on (e.g. DSC1001.jpg) and write below line in the command prompt:
   convert file_name0001.jpg -resize 600x600 -background black -gravity center -extent 600x600 square_output.jpg

   • You can find the a description of the options in the convert manual. Here are direct links to the options used in the current command:-resize, -background, -gravity, -extent

7. Adjust the options as required. When you're satisfied with the end result you can run the command in a loop over all jpg files. In below command I use the same option values as in step 6 as an example.
   FOR %a in (*.jpg) DO convert %a -resize 600x600 -background black -gravity center -extent 600x600 square_%a

   • This command loops over all files in the folder of step 4 that end in .jpg.
   • Every file that ends in .jpg is put through the ImageMagick command of step 6.
   • After processing the ImageMagick command prepends square_ to the filename and saves the processed file. The original file remains untouched.