There is a faq about this (see below) http://plantuml.com/faq
Images cannot be deleted but they don't have to be deleted since they are not even stored on the server.
Tell us if it's not clear enough for you. Thanks!
How long do the images generated by PlantUML Server live for?
Links to png or svg generated by PlantUML Server are valid forever (that is as long as the server is up). However, we do not store any diagrams on our servers.
This may sound contradictory. It is not: the whole diagram is compressed into the URL itself. When the server receives the URL, it decompresses the URL to retrieve the diagram text and generates the image. There is no need to store anything. Even if the server is down you can retrieve the diagram using the flag
-decodeurl with the command line.
Occasionally we may activate HTTP traces on our server. This is mainly for performance issues (when we have some) to understand the traffic we get. Once the issue solved, we turn back off HTTP traces and we remove the logs.
Note that we are also counting the number of diagrams generated (printed at the home page to measure general server load).
Concerning sensitive contents, even if we do not store generated diagrams, please be aware that all the traffic goes through HTTP, so it's easy to catch.
So you should probably install a local server on your own network if you plan to generate diagrams with sensitive information.