Image Background for Strip

I’m planning on creating a custom image to use as a strip background. Although the strip is stretched, I don’t want my image to tile. So, a couple of questions please: What width/height ratio should my picture be, and is there a suggested pixel dimension? Also, is png, jpeg or svg the preferred file type?

Thanks.

https://support.wix.com/en/article/adding-an-image-to-your-strip