You have a logo or image and need to create app icons for iOS and Android. Doing it online is the fastest way: upload once, get every size and format the stores need.
Step 1: Prepare your image
Use a square image, at least 1024×1024 pixels. PNG or JPEG is fine. Avoid tiny text or thin lines that disappear when scaled down.
Step 2: Upload and choose platform
Upload the image to an app icon generator. Select iOS, Android, or both. Some tools let you set the base filename (e.g. ic_launcher for Android).
Step 3: Download and add to your project
Download the ZIP. For iOS, add the AppIcon.appiconset to Assets.xcassets. For Android, copy the mipmap folders into res/. You're ready to build and submit.