Grey bar on grid content image

I am having an issue when uploading pictures onto the grid content area. There is a dark grey bar across the bottom of the large image that will only go away when I select one of your sample images. I have tried changing the sizes of the images I am uploading to exactly like your sample images but the bar is still there. How can I get rid of this? https://itblogs.semo.edu/ Here is the link to my page. Thank you