Hmm ... where did you rename it?
What I meant was the following:
- Copy your image to your scene folder (you can use ProjectMans import-function therefore or just copy it by hand)
- Open your scene in SceneEdit (just double-click it in ProjectMan)
- Select the node named "Background"
- and in the properties, change the background-picture by clicking on the button behind "Sprite"
Have you already worked through the tutorial in the documentation?