How may I use a photo which is on my computer, for a search by image? I have already tried to upload it to my Google+ Unlimited account.
Use Chrome as a browser
Go to www.google.com
Click on the Images link (currently on the right side near the gear wheel etc)
Click on the camera icon that is now in your Search bar
Click on the Upload an Image tab
Click on Choose File: this opens a file navigation window - on it, choose the file on your computer.