You can use Google Chrome developer tools to find out the link to your apps graphic in the google play developer console.
The link to this graphic is like that: https://lh3.ggpht.com/tl0FC9sBdKWSI37JejuhKrMjlrZkZNbAJmDhaYvs6WahtRvNZC6XEiTIcyQiJ7a2QQ=h300-rw
As you can see there is “h300-rw” at the end of this link. It is the dimension of your image. If you change that for example to “h1024-rw” then you’ll get a hi-res image:
https://lh3.ggpht.com/tl0FC9sBdKWSI37JejuhKrMjlrZkZNbAJmDhaYvs6WahtRvNZC6XEiTIcyQiJ7a2QQ=h1024-rw