I had something similar happen to me when I installed UNA and then upgraded the PHP version AFTER the install. You can try to do this:

Go into Studio > Settings > General - Remove the chechmark from "Use GD library for image processing". 

From Studio launch page click Dashboard. Click the down arrow by Cache and delete all cache.

Refresh your website page and see if the images display. 

IF THEY DO - Go back into the General settings and put the checkmark back next to "Use GD library for image processing". Clear cache and refresh your website page and they "should" still appear. 

This worked for me when I had the same issue. I figured it was due to the caching of how the images were stored/rendered after the upgrade of PHP.