An SVG is all you need

>They are supported on just about every device and platform

Except they aren't. I recently used a simple SVG in a background and Safari wasn't able to render it properly so after trying lots of different things I gave up and used different sizes of raster images instead.

SVGs also are mostly unsupported for og:image tags (dependents on the app/browser). I know it's supper specific and I am not even sure if open graph is standardized protocol or not, but it's used everywhere.