Bannerbear is described as 'helps you auto-generate social media visuals, ecommerce banners, dynamic email images and more with our API and integrations'. There are three alternatives to Bannerbear for Online / Web-based and SaaS. The best alternative is APITemplate.io. It's not free, so if you're looking for a free alternative, you could try HTML/CSS to Image or Pixelixe.
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.