Compared to Apple App Store, uploading your own app screenshots to Google Play Store certainly is much easier.
Updating or uploading your own screenshots to Google Play Store does not require a new app binary from Google, so you can go ahead directly to app to do this.
You will need to prepare at least two sets of screenshots with different sizes/dimensions. The images need to be either PNG or JPEG, with no alpha or transparency.
Unlike Apple, Google doesn't really have a specific requirement for the screenshot dimensions. The only rule regarding the dimension is that the image has min length of 320px for any side, and max length of 3840px for any side.
Just for your references, for phone screenshots, we usually use images with dimension 1242 x 2208. While for tablet screenshots, we usually use images with dimension 2048 x 2732.
Once you have the images ready, you can go ahead and sign in to your Google Play Console account. Select your app, and go to the Store Presence and then Store Listing:
Scroll down until you see the field to add your screenshots. You are only required to provide screenshots for Phone and Tablet. But of course, feel free if you want to add some for Android TV and Wear OS :)
To upload your screenshots, you can simply select the Phone/Tablet tab, and then drag your images or click Browse Files:
If the screenshots are uploaded without any issues, there should be a pop-up message at the top of your page saying 'Upload Success'.
Once you've successfully uploaded your screenshots, you can go ahead and click the Submit Update button:
That's all! Google usually only takes a few hours to get your updates live in the Play Store.