Open the downloaded image in your Gallery/Photos app > Set as Wallpaper/Lock Screen. A Note on Conservation
Look for 1920x1080 (Full HD) or 3840x2160 (4K) . For Smartphones: Look for 1080x1920 or higher. elephant screensavers free download
When searching for free downloads, it is important to use reputable sites to ensure you are getting high-resolution images without bundled malware. Open the downloaded image in your Gallery/Photos app