I've been seeing this and scratching my head for a while. For SEO purposes, the window's title is "Microsoft Office Security Options" and the key bits of the message are "references to external pictures have been blocked" and "Warning: this document references pictures in untrusted locations. These references have been blocked".
I wasn't able to disable this alert completely but added a "trusted location" for the path where I store files for my services (C:\Services\). You might also want to add C:\Users\...\AppData\Roaming\openlp\data to cover any Powerpoints that OpenLP makes a copy of.
