Avoid using copyrighted images for your profile picture. Either use licensed images or create your own to avoid potential legal issues. Also pay attention to the license terms if you use images from third-party websites.
Yes, you can restore a previously used profile picture. Go to your profile options, select "change profile picture" and scroll through your saved pictures to select a previous profile picture.
To protect your profile picture from unwanted downloads, you should choose a private profile and make sure that only your approved followers can see your profile picture.
A recommended app for zooming profile pictures is XYZ Zoom. It offers a user-friendly interface and good image quality.