Can't Get a Perfect Square

I can't for the life of me get a perfect square. For example, I'll set the dimensions to 1200x1200 pixels and then take a screenshot with 1Click Screenshot (or manually using CMD+Shift+4), but when I go to inspect the file properties in Finder they'll be 1200x1067 pixels. 

Does anyone have any idea what might be causing this and how I can fix this? Thanks in advance.