SDF Texture Generator by AtomicJoe

SDF Texture Generator by AtomicJoe
http://filterforge.com/filters/15060.html

You "can", but it will not make a good heightmap because, as you can see, it scales a pixel to a square instead of a circle (which isn't very natural) and the greyscale doesn't have a lot of precision sadly (since each grey step is made manually because of how filterforge works).
You can try to blur the generated SDF texture using Gaussian Blur. That would fix both issues but not completely.

That filter was specifically made to generate SDF Textures easily from any image, since it's very useful for realtime engines and doesn't seem to be a lot of options to generate this kind of textures and keep them seamlessly tileable.
For the best results, use a high definition image as input and import the generated SDF texture as a quarter of it's size in the 3D Engine (downsampling it in a regular graphic application could break the seamless texture)

You can read more about SDF Textures here:
Original Valve Paper

And a specific implementation for Unity here:
SDF Texture Generator for Unity

SDF Shaders for Unity

This implementation is much better than this filterforge filter, but doesn't generate seamless textures.