One of the few times I've used diagrams were some entity-relationship (ER) diagrams of some databases that were generated by a tool and I found them pretty useful. They helped me get a big picture understanding of the database and see relations much easier. It also helped me notice some small issues with the database that I don't think anybody had noticed like some orphan tables and missing foreign key references. Putting static images in documentation might not be ideal if the database changes quite a bit, but it might also not be the worst if you put the caveat that they might not be up to date. They're easy enough to regenerate when you want an up to date ER diagram of the database.
I've also seen a single incomprehensible diagram used as the only documentation for an entire existing system or a system to be built before which was pretty frustrating. That leads me to think that diagrams should probably aid and be used in addition to other documentation/explanation rather than be the sole explanation.