Steps:

• Convert to grayscale: Menu > Image > Mode > Grayscale
• Adjust coloring levels, optimize the desired contrast: Menu > Colors > Levels
• Edge detect: Menu > Filters > Edge-detect > Difference of Gaussians
• Set mode to RGB: Menu > Image > Mode > RGB
• Add alpha channel: Menu > Layer > Transparency > Add Alpha Channel
• White to transparent: Menu > Layer > Transparency > Color to alpha
• Export the image to PNG
• Create a vector image from the result PNG, on e.g. http://www.vectorizer.io/
• Print the SVG on the desired size!