I see a lot of pictures of vehicles held up by floor jacks during tire changes. Isn't that unstable and unsafe? Seems like a risk to the shop workers as well as possible damage to the vehicle.

As a master diesel technician, jacks are perfectly safe for short duration repairs where the tech is not under the vehicle. If going under the vehicle or it's going to sit for an extended period then jack stands are a must.

I second Frank Diesel's response.

THERE ARE JACK STANDS UNDER THE VEHICLES ALSO FOR SUPPORT.

Yeah it is don't know why they even do that.