sorry, my google is in french :3

Step One :

choose your picture ▲
copy the link of your picture ▲

Step Two :

go to google image ▲
click to the little camera ▲
click to seach 

Step Three :

scrool down
click to the picture that seems to be the original (it may not be the original size) ▲

That's it!

ʟᴏᴏᴋɪɴɢ ғᴏʀ ᴛʜᴇ ᴏʀɪɢɪɴᴀʟ sɪᴢᴇ ?:

repeat the step one and two with the link of the picture that seems to be the original
than click to "all size" ▲
you'll find a lot of versions of the pictures with different size ▲

sometimes, you have to do the step 1, 2 and 3 more than once to find original picture.

DONE !