Add a background to Chrome Web Store screenshots

[mcintyre94]

The extension screenshots in the Chrome store look a bit rubbish because of the transparent background and required size for the store. Probably worth adding a nice gradient background, or text!

Phantom uses text + a background: https://chromewebstore.google.com/detail/phantom/bfnaelmomeimhlpmgjnjophhpkkoljpa

Solflare similar: https://chromewebstore.google.com/detail/solflare-wallet/bhhhlbepdkbapadjdnnojkbgioiodbic