Pages '09 v4.3 is working on my OS X 10.11.6 installation. How did you originally install Pages '09 before the El Capitan update?
If you purchased Pages '09 as an individual application from the OS X App Store, it is a self-contained application, and can be restored from your backups. If you applied the Apple iWork 9.3 updater to this App Store purchased Pages '09, then it is destroyed. If you originally installed from the iWork '09 DVD, then use that, and follow that installation with the DVD-specific Apple iWork 9.3 updater. That will get you to Pages '09 v4.3, the last release version.
If you no longer have the iWork '09 DVD, then you can purchase a new one from Amazon or Ebay resellers, and then apply the preceding linked Apple updater to that installation.
Your last sentence. Once you have selected the shape, clicked on the Style tab, set it to Fill : Image Fill : Scale to Fill, and Choose... your image — you are done. You should have a shape with an image background — operative term here is background.