It is showing that I have successfully uploaded a photo (I tried PNG and JPEG with same result) but then it is just showing a big gray box. Is there a setting I'm missing?

Would recommend switching to an unmodified theme and trying again. If the unmodified theme does not have an issue, it likely your modified theme.

I'm not sure how to change the theme on the events page itself, since it's created within the site, and not in the admin space. But the events page I was trying to set up to add this block to is the default theme. 

I clicked to edit the page and it suddenly showed up. That is very bizarre. Problem solved I guess?

6 minutes ago, AdminHaptX said:

I'm not sure how to change the theme on the events page itself, since it's created within the site, and not in the admin space. But the events page I was trying to set up to add this block to is the default theme. 

You would need to create an unmodified theme in the ACP -> Customization -> Themes. Ensure it is available for your member group, then scroll to the bottom of the page on the front-end and switch to the unmodified theme.