Looking at the URLs you provided above have https on them. The Pronto does not do https, only http. The URLs I use below are http only and will work on the Pronto.

I just downloaded the first link's avengers image from fanart and the size on disk is 947KB and the image is 1000 x 1426. I also noticed that the download was a progressive JPG file which likely downloads 2-3x the size of the final image. The 2nd link you provided is a non-progressive JPG with a size on disk of 596KB and the image is 2000 x 3000.

After this analysis, I figured I'd browse the site from the first URL.

I used Chrome and started inspecting the html they were using. Sure enough, they have a URL that supports resizing (thumb.php). You just gotta know how to use it. The URL below downloads the same picture you posted but resizes it to 200x200 prior to download. This saves a tremendous amount of size. The height/width is 200 and the file size went from 976KB down to 28KB.

ht_tp://assets.fanart.tv/thumb.php?src=/fanart/movies/24428/movieposter/marvels-the-avengers-5218f1301e057.jpg&a=t&h=200&w=200

ht_tp://assets.fanart.tv/thumb.php?src=/fanart/movies/24428/movieposter/marvels-the-avengers-5218f1301e057.jpg&a=t&h=1426&w=1000

This URL renders a top-down JPG file in the same dimensions as the original but the file size is 596KB.

The Pronto does not have very much memory so therefore, I'd recommend using thumb.php from fanart.tv to download in the size you would like.

ht_tp://assets.fanart.tv/thumb.php?src=/fanart/movies/24428/movieposter/marvels-the-avengers-5218f1301e057.jpg&a=t&h=210&w=139

This yields a file size of 21KB so HTTP data won't be much more than that.

As a test, you can flip the width and height and retrieve link from a web browser and you will see that the php file crops the original image as needed.