Site to Image is described as 'Effortless Website Screenshots. Give our API a URL and we'll give you back a screenshot' and is an app in the Photos & Graphics category. There are four alternatives to Site to Image for Online / Web-based. The best alternative is CloudBrowser.co. It's not free, so if you're looking for a free alternative, you could try HTML/CSS to Image or Thum.io.
Simple API to convert your HTML & CSS into shareable images. Supports HTML5/CSS3, renders images exactly like Google Chrome. Built in CDN, image resizing, custom fonts and emoji support. Works with any framework or language.