Unfortunately most game feeds don't have very large thumbnail images. The only option would be to take your own screenshot of the game if you need higher resolutions.
To do so follow the steps below:
1- Go to Chrome extensions and download Awesome Screenshot. Press ADD TO CHROME button and let it install.
2- Go to your website url and open the game page you want to take a screenshot from.
3- Press the colored lens icon in your toolbar. (Choose : Capture Visible Part of Page)
4- Use the crop function at the beginning of the toolbar to crop the picture, select the area you want to crop and drag the corners to enlarge or to make smaller.
5- Click crop to crop the image.
6- Press "done" once you're ready editing the screenshot.
7- A pop up will show and you'll see a preview of the screenshot. You can either choose the option "Save As" or "Upload to an online photo storage".
8- Go to YourWebsite.com/admin/games -> Edit game
9- Then Go to Thumbnail section Upload your thumbnail or Grab it from an url
