I think DrewDad is pretty much right.
If you use someone else's art from the internet without permission in a way that earns lots of money, they can sue. They will certainly be able to force you to remove their work (and I am not sure if in a commercial context they might not be able to get money).
But in a internal newsletter, the worse that could happen is a cease and desist letter. This is highly unlikely for lots of reasons... not least of which nobody cares.