/layer/ is an experimental board for desudesutalk, usually shortened to DDT. DDT is a userscript which allows arbitrary text to be embedded in images. As such, text is not allowed on /layer/.
To use DDT, you need Greasemonkey, also known as Tampermonkey on Chromium and Violentmonkey on Opera. Once that's installed, use this link to install the script: https://github.com/desudesutalk/desudesutalk/raw/master/ddt.user.js
To make sure everything works, navigate to a thread and head over to the "messages" tab. Click "get old messages" and the script will attempt to decode messages in the thread. You will probably not have anybody sending messages you can see at first. When you write a message, it is signed in such a way that only people in your contacts can see it. If you have just generated a key, nobody has you in your contacts, and you will not be able to see messages.
You can add a public key to your contacts and give that person a name by clicking the "add" button under an image they have posted. Once you have some people in your contacts, you can start posting messages to them.
That's the basics. A slightly more detailed guide can be found on Github at https://github.com/desudesutalk/desudesutalk/wiki/How-to-use-this-script
, but aware it is somewhat difficult to read.