How can I retrieve the profile picture that I have on my Gmail account? The original is not in my phone anymore so I would like to download it back so I can have the picture in my smartphone again.
Just open the page which has the photo, Press "Print Screen" - a screen-shot of the screen will be copied in your clipboard. Then, go to MS Paint, paste it there crop it and save.
As mentioned by Bibhas in comment, Go to your Google+ profile. Click on the profile image there. It should give you the option to download. If you do not have a Google Plus account, just create one, it will be created by default with that image, then you can download the image from there and delete your Google+ Account (if you wish).