Default image for Advertising banners

Hope someone can help.

I’m using the Classifieds theme and utilising the Advertising Sell Space functionality.

I’d like to replace the default “Advertise Here” with “view pricing” default display for default images, please can somebody point me in the right direction to set this up. I’m using the Header, Sidebar Left Bottom, Sidebar Left Top spaces and have created default images but not sure how to set them up.

Many Thanks in advance for assistance.