The standard size for images displayed on articles is 250px. If you are interested, I can show you a couple of good websites that will give you access to any screenshots from any movie/show you can think of. All in original format and high quality.
If you have bothered to check, we have subcategories for certain specific pages. Example; for the Princess Elena page you only need to insert the "Elena of Avalor characters" category and nothing else. Every category has a source that you can go to. Look them up.