Yes, all of these Zoom apps are available for free. You can download it from the App Store (for iOS) or Google Play Store (for Android).
Yes, these Zoom apps are usually designed to be user-friendly and offer an intuitive user interface. You shouldn't have any trouble enlarging your profile pictures with these apps.
When using high-resolution images and a high-quality zoom app, the image quality should not be affected. Make sure you use an image with enough resolution for best results.
Or maybe you just want to take a closer look at someone's instagram profilbild to get a better impression of them.