Q: Modify El Capitan (10.11.4) Guest Account

I have exactly same question. At Tillamook County library, we currently have 1 IMac for testing. I am trying to setup this IMac to have customized dock. A lot of those apps on the dock that come by default the kids will not use, and we would like to put kids programming apps on the dock for easier access. The background image isn't important to us but that's something we would like to change as well. I know we can create standard account, but the whole point of the Guest account is erasing everything at the end.

The first thing you should do is install 10.11.6.

That is done. I'm on the latest version.

Then start your own thread instead of tagging onto a thread that is talking about a different system than yours.

Writing an effective Apple Support Communities question

It's still El Capitan. It's not different system, but different version. Why create extra thread when there is one already opened. If you have nothing logical  to say other than trolling then please move along.

OK. Good luck. Hopefully someone will come by this old thread. At least you should read the link I posted. We have no idea what the details of your system or problem are. You have posted nothing that would allow us to help you.

whole point of the Guest account is erasing everything at the end.

I think you have answered your own question. All changes are removed. If you want to permanently alter the guest account, you are going to have to do some system hacks, which in the long run might affect computer operation.