1)

Go inside the Workflow Guide and click on "Add Step"

2)

Click on "Image Step"

3)

Click on "Upload Image"

4)

Click on "Open" once you're done

Select an image to upload

5)

Scroll the creator panel down, for "Resize" option and click it

6)

Image is resized!

You can start editing the image step using tools, available in the creator panel