The images included in a grid template can't be deleted apparently. The option to delete image should appear on the left of the screen under "Image" when selected. See screen shot.
Thanks for reaching out. If you're trying to delete an image you can select it and press the delete button. Also, you can add your comments here https://adobeexpress.uservoice.com/forums/951181-adobe-express if you would like to raise a request for it, as this page is being monitored by our product team.