This is directly possible in WriteMapper as of the release of version 2.3.1.
You can directly export your mind map as a PNG image file from the WriteMapper app itself, no multi-step workarounds needed. From the export button in the toolbar, you'll find the "Export Image" option available, which does exactly that.
Do note that the size of the image will correspond to the zoom level of the mind map that you have configured in the app, and that states of nodes being collapsed or expanded will be mirrored similarly in the exported image as well.
With the image of the mind map saved to your desired location, you can then choose what you want to do with it, such as editing it using your graphics editor of choice.