Just a couple of questions that might help with answers.
What image format do you normally sent to the site - png - jpeg -tif ...or..something else?
What computer operating system do you use - Windows 7 / 10 - Linux - OSX ? That might determine any plugins etc that can be used.
For Windows there is a batch plugin BIMP that resizes https://alessandrofrancesconi.it/projects/bimp (I can maybe help with linux if you use that)
What image format do you normally sent to the site - png - jpeg -tif ...or..something else?
What computer operating system do you use - Windows 7 / 10 - Linux - OSX ? That might determine any plugins etc that can be used.
For Windows there is a batch plugin BIMP that resizes https://alessandrofrancesconi.it/projects/bimp (I can maybe help with linux if you use that)