There are so many things that could be going on that it's tough to help you without actually seeing the workbook.
Can you mock something up that illustrates your issue?
Original post updated with workbook (I realized there's no sensitive data in it).
Cool. I’ll look shortly.
Is there a reason these need to be sheets? Why not just use an image object? One additional problem with doing these as sheets is that shapes don't have great resolution, so the quality of them won't be great. If you absolutely need them to be sheets and not an image object, then I'd suggest leveraging background images instead of custom shapes. You'll have a bit more control over the size and you shouldn't lose quality. The following blog talks about using background images to swap between different images, but the steps to create a background image should show you how to use them as I've described: https://www.kenflerlage.com/2018/01/image-swapping-in-tableau.html
I read this article when I was first looking at solutions and would love to use images. However, the requirement is to be able to click on the image and navigate to a second dashboard. I supposed I could use images with URLs navigating to a different workbook on Tableau Server but was hoping to do it all in the same workbook.
Okay, that makes sense. I'd suggest doing two things:
1) On the dashboard, change the sheets to Fit Entire View. This will center them and allow them to expand nicely.
2) On each sheet, increase the size a bit so they fit nicely in the window on the dashboard.
Note: because your images may not be the same dimensions as the objects on the dashboard, it will likely crop the top/bottom or left/right of the image somewhat.
Wow so simple but it works. I was manually resizing the pic on the sheets using the arrow keys and that did not have the same effect.