Once you have signed up for a Google Play Developer Account, you’ll start filling in the information about your app in the Store Listing which includes text and images.
Each part of the store listing has length/dimension or other requirements. Make sure to follow these.

Graphic Assets required for Google Play Store Listing:
The minimum graphics are 2 screenshots, a high res icon and a feature graphic as in the guidelines below. More screenshots, a promo video and a banner are optional.

Some of the basic requirements are listed below, however the most up to date requirements are available on Google Play Console Help.

Screenshots

• Minimum dimension: 320px
• Maximum dimension: 3840px
• The maximum dimension of your screenshot can't be more than twice as long as the minimum dimension.

High res icon

• 32-bit PNG (with alpha)
• Dimensions: 512px by 512px
• Maximum file size: 1024KB

Feature graphic

• JPEG or 24-bit PNG (no alpha)
• Dimensions: 1024px by 500px