In your image the object (ROI) and image is clearly separable, you can directly apply the imbinirize function on the input image.
Also you can set the threshold own limit (range 0 to 1, say 0.4 etc), and see the results, test with different threshold values and check the best result in your case. In addition after binary operation, you can do some morphological operation to smoth the boundary and segment ROI blobs on binirize image to extract the region of interest (not always).
Already I have mentioned, in your case the hand is clearly separable, just try it. If you will find any difficulty, let me know.