You have 900 images to process, you need some sort of batch processing.
Edit: question: What format are the images ?
It can be done using Gimp script-fu but not easy for regular users.
There is a Gimp plugin BIMP for batch processing. A Windows installer here: https://alessandrofrancesconi.it/projects/bimp/ There are examples on the BIMP site.
You need to use "other Gimp procedure" with a "threshold" setting and values as this:
There is another procedure to add for saving in png format.
Add your folder of 900 images, set an output folder and apply. There are some restrictions. Original image format png / tif are ok. Format gif gives a black image.
Edit: question: What format are the images ?
It can be done using Gimp script-fu but not easy for regular users.
There is a Gimp plugin BIMP for batch processing. A Windows installer here: https://alessandrofrancesconi.it/projects/bimp/ There are examples on the BIMP site.
You need to use "other Gimp procedure" with a "threshold" setting and values as this:
There is another procedure to add for saving in png format.
Add your folder of 900 images, set an output folder and apply. There are some restrictions. Original image format png / tif are ok. Format gif gives a black image.