There are two steps required in order to display your logo:
Upload your logo
Update your reminders to include the logo placeholder
Upload your logo
The first step is to upload your logo
Click on your organisation name (top right), then
Click Settings, then
Click Reminders, then
Scroll down to the "Organisation Logo" section, then
Choose a logo image file from your computer, then
Click Upload.
Add the logo placeholder to your reminders
Once you have uploaded your logo, you need to update your reminders to include your logo.
Click on Workflows in the main menu, then
Click on a workflow, then
Select a step, then
Click Insert {Placeholder}, then
Choose the size logo you want to add, then
Click Save.
Repeat this for all your workflows and reminders