App need to resize image with transparent background

We need an app built that will loop through a folder of .jpg and .png images. Each image needs to be resized (keeping the ratio) to where the longest measurement is 400 pixels. So if the image is taller than it is wide, the height would be resized to 400 pixels and the width would be shorter, based on the original image ratio. Then the app needs to make the resized image 400x400. So, if the height is now 400 and the width is 200, we need 100 pixels added to each side of the image, making the width also 400. This added size needs to be transparent and the resulting 400x400 image needs to be saved as a png file type. If both the height and width of the original image is less than 400, the image needs space added to both the height and width to get it to 400x400 (still transparent background). The app should take in three inputs: 1)folder path of where the original images are and 2) folder path of where the new images will be saved and 3) resulting height and width measurements (in pixels and defaulted to 400x400). This app needs to work without needing photoshop. Please ask any questions.