If you've been trying for years to solve this issue going as far as communicating with the developer of a plugin (workaround) and still no solution? Maybe it's time to move to another client?
I know that's not very helpful, but it might be the right thing to consider.
Edit:
I thought of one more thing. In Outlook, a new sender that hasn't been marked or added to the safe senders list will block pictures similar to this. Does Pegasus have such a function?