yFiles for JavaFX uses JavaFX as the rendering engine and JavaFX unfortunately does not directly support SVG images. You need to convert the SVG image into an image that JavaFX understands and then you will be able to use it with ImageNodeStyle
.
Using WebViews would be another option, however this is a very heavyweight solution that does not scale very well for many SVGs. You can surely use for rendering a background SVG, but I would not use hundreds of instances to show simple SVG graphics for the visualization of nodes.
Take a look at this answer to a related SVG/JavaFX question: it proposes a solution to the generic problem, which should also work for this specific issue.