There may be several situations regarding "Delete images":
1. Remove an image from a question:
Click on that question and then click on the image. Press Delete or Backspace on keyboard.
2. Remove an image from a choice:
Click on the image button after the choice, click [Clear Image] button on the bottom left corner of the pop-up window.
3. Delete an image from Gallery:
Go to Gallery page, find the image you want delete forever. Click on [Delete] button under the image.