Client support people communicate to your current clients hundreds of times a day. Use that space for upsell, upcoming events, cross-product promotion, etc. Have the image feed in dynamically and you can easily chang it out without bothering the client-facing folks.
Note: keep the message light and informative. Soft-sell. The receipients KNOW you already. Keeping the message fresh makes the cleint-facing folks feel like they are delivering content, not just an ad.