Remove an image from a gallery block

Site URL: https://www.advent.church/#leadership

I'm trying to remove one of the images from my gallery block. There has to be an easy way to do this that I just can't figure out. When I go to edit the page, and then the gallery block I can upload pics or change the design to a slideshow, etc. but I can't figure out how to delete just one image and add another. 

It shows this gallery block as having a name, so maybe it's set up somewhere other than this page and I need to go find it and edit it but I don't know where to look.

It is a 7.0 template so you go to the page with the block, click page edit then click the  block hit the edit pencil icon there, then make sure you are on content, then hover over the thumbnail of the image you want to remove, then click the trash can in the menu above the thumbnail. Scroll to the bottom of the thumbnail display to find the upload images arrow in a  thumbnail block or click the plus sign down in the right corner.