Click the "Insert Image" button on toolbar.
A window for "Insert/edit image" is shown. Enter URL of your image into the "Image URL" field. Alternatively, you can insert an image from your PC, by clicking "Browse" button in the window, as circled in red color below.
NOTE: The file size of the image can't exceed 180KB.
A window for "Upload image" is shown. Click the "Browse" button and select an image file from your PC.
Then, click the "Upload" button and the image will be ready for your use.
Click the "Insert" button in the "Insert/edit image" window and your image will be shown in the text area of the message body.
NOTE: In order to provide information to user with disabilities, please input the desciption for your images.