I know this is an old thread.
I'm 100% guilty of changing in a bathroom stall. I'm not comfortable changing in the open period (years of body shaming by my mother and being taught my body is something to be hidden and ashamed of). My gym offers two dressing rooms (similar to a store) for changing. If there's a line, I do go to a bathroom stall. Even at peak times, I've never seen the stalls all filled (10 stalls).
The triceps machine is not the selfie machine. It is the only such machine in the gym and it is not polite to delay someone's workout because you want to post a selfie to social media. Sure, take a quick one, but don't post from the machine. If you must post from a machine, use one that is not the only such machine available.
If someone is wearing headphones, do not try and make conversation with them.
After you wipe down the machines, please use the garbage cans available to properly dispose of your paper towel. Don't leave it in the cup holder for someone else to deal with.