A nice plugin by Kanahiro Iguchi.
- Set Project CRS as WGS84(EPSG:4326).
- Set Accuracy and Color Threshold, and Click mapcanvas where you want to make polygon.
- Single Mode enable you make ONE polygon around where you clicked.
- Higher Accuracy needs so longer time that I recommend you use this plugin in Low Accuracy.
- To make polygon apropriately, it is better adjust Saturation highly.