Cannot remove image placed in iMovie timeline

Hi, I have put an imagine in my iMovie timeline & stated editing it, but have realised I need to replace the image with another one however when I right click the image, there is no delete button. When I select to delete the imagine in the 'my movie' section with all the other clips, its saying its 'in use' so I can't delete it until its deleted from the timeline? But I have no idea how to actually delete it now..