From time to time you may see gaps appearing on the website in between the Movies that appears there. This is because the posters are not cropped to the 2:3 ratio that they need to be. To check which posters need cropping, go to the Movies section. You can see there are some posters that have white spaces in this section (shown below):
Then, go into the posters section in the Movie. In this example I will do Black Panther.
Go to 'Download from TMDB' and find the same poster you had before. Then, on this screen, choose the button next to the delete button. This lets you crop the image.
Then, click the 2:3 ratio button to bring up the blue cropping window:
Drag the window so it covers as much of the poster as possible, then move it into the middle of the poster and click save.
Wait for the image to save, then click the bin icon on the old poster (it will be on the left) to delete it:
Then click the anchor icon on the remaining poster to make it the default poster that appears on the website: