Question

Pressing and holding the what key while dragging a corner handle of an image proportionally resizes it

Answer 1

The left key I believe. I have a Mac so 

Answer 2

Hey there!

Holding down the "Shift" key allows for you to resize your document or photo freely without distorting it in any way. It will stay the exact same ratio as before you resized, only the document itself will be proportionately larger. 

Hope this helped you out! :-)