Old thread bump, I know, but I ran into this today and have a solution which is especially helpful if the listing is no longer showing in SERPs, but you still have access to the listing in GMB dashboard. The steps listed below are done in Google Chrome so you may have to tweak the process to work with your setup.
1) Log into Google My Business and go to the listing in question.
2) Once you're in the listing, click Photos in the left nav.
3) With the photos now appearing as thumbnails bring up Chrome's Inspect Elements panel by right clicking and choosing Inspect, or from the browser's main navigation bar (View -> Developer -> Inspect Elements on MacOS).
4) With the Inspect panel up, use the Select tool (the icon of a square with an arrow inside it found in the top left corner of the Inspect Elements panel) and then click one of the pictures in GMB.
5) This will highlight the image URL in the Elements section of the Inspect panel. Right click the highlighted URL and Open in new tab.
6) You'll notice that the image opened as a thumbnail and not the actual size. In order to get the actual size, remove everything after the =s at the end of the URL and change it to =s0 and reload the page.
7) Now you have the image in full size and when you save it, you'll also keep the original file name from when it was uploaded to the account.
Hopefully this was helpful for future searches.