To convert an entire folder of images into different formats on Windows one of the easiest ways is to use
Softaken Image Converter Software. After installing, just launch the program and add the folder containing your images. It supports bulk conversion so you do not need to process files one by one. Choose the output format you want like JPEG, PNG, BMP and GIF, set the destination folder and click Convert. It is quick, user-friendly and does not compromise on image quality. I've used it to convert batches of images and it works smoothly.