You don't need Photoshop or some other advanced piece of software to resize a large number of images at once. You can use Microsoft Office Picture Manager which is part of the Office suite.
Tasty drinks like our grandmothers used to do it, they were tasty, easy to make and full of vitamins, let's make a smoothie out of oranges, bananas and apples.
Knowing how to select parts of your Gimp documents with the help of Quick Mask might be one of the first things you should learn when starting with Gimp. Let me introduce you to the basics of Quick Mask.