If everything else fails :
- Google for Corel Draw trials
- download CorelDraw Graphics Suite X7
- install & start it
- drag your picture into it
- in the menu, choose 'bitmaps', then 'mode', then 'Black and White (1-bit)'
- you'll get a window, choose 'LineArt' as conversion method, set the threshold to 108 by moving the slider or clicking in the box and typing it in
- click on 'ok'
- you'll get a pretty jagged picture on top of your original
- move the mouse pointer on the picture, right click and choose 'Outline Trace' and then 'High Quality Image'
- you'll get a window, wait a bit and then click on 'ok'
- you now have a vector image that you can resize without quality loss
- you can save the pic by going into the 'File' menu, then 'Export'