Copy png file to the template directory using SFTP.
Template directory /usr/share/remarkable/templates
Edit templates.json
e.g
{
"name": "Comic (4 frame)",
"filename": "Comic2",
"iconCode": "\ue98c",
"categories": [
"Comics"
]
}
Note don't include extension in filename (e.g don't add .png)
Restart device:
systemctl restart xochitl