Refer to the Creating an OSM Message solution to begin.
Click the "Use Your Own Image" template to upload a jpg. Click the Next button.
Note: For this template, the only upload-able format is a JPG.
Name your message and set a duration. The message duration is automatically set to "Unlimited", so be sure to set a proper duration. See below, 10 seconds. Upload a background image and click preview. If happy with the image, click Save. The same process applies to Side and Banner zone message creation.
Once the message is saved, it will appear within the zone message folder. OSM provides message details to the right that give information such as duration and when the message was created. See below.