No third-party apps, no risk of malware, free. Cons: Very low resolution (screen resolution, ~72 DPI), includes UI elements like the "X" button or contact name, and you cannot get the original file.
Take a screenshot. Most modern smartphones will save this directly to your Photos or Gallery app . viber profile picture download
You get the original, unmodified image uploaded by the user. Cons: No third-party apps, no risk of malware, free