1) Go to your app's iTunes Preview page on your browser
2) Choose a great screenshot, right click, and save it using "Save Image As..."
3) Open the image in Preview (Mac users)-->Tools-->Adjust Size-->600w x 360h-->Save
4) Your image can contain upto 20% text. To add text to your image, try one of these tools
Here are some best practices to improve your Mobile Ads' response rate