I opened imagemagick through the terminal with $ display filename.jpg

Once it opened, the terminal was in use (?) as in no more prompt, just blank space where I could type anything like when you write to a file with 'cat'. I used CTRL + Z to get back to the prompt and the image stayed open in imagemagick. Now I can't close it - clicking the red X on the window doesn't do anything. How do I close this window/program?