Sounds like SVG is not supported in the Emacs build you are using. From the Spacemacs comment for the banner image:
A string value must be a path to an image format supported ;; by your Emacs build.
Try a different build of Emacs, or use only supported image types for the curry build being used, i.e. png
I have a little example project I will share soon in the Practicalli GitHub repository
ha. got it.
png did the trick