Spark Application does not have a backgroundImage property. Are you using Spark Application or Halo Application?
Peter deHaan has a great blog post on how to set a backgroundImage in Spark Application: http://blog.flexexamples.com/2009/03/22/setting-a-background-image-on-an-fxapplication-in- flex-gumbo/
I think it should be:
If your background image size is large then don't Embed it so that it will add to your SWF file size, you can use the same way you are specifying it in the css but you also need to specify the background-size property along with the background-attachment property.
MainApp is the root
MainApp/src/main.mxml - (application file)
MainApp/images/bImage.png ==should be => MainApp/src/images/bImage.png
No need of giving the full path...MainApp/src/images/bImage.png
Only the relative path is enough and recommended.
The issue is not the "path string" in CSS, but the image physical directory.
The image file should under "MainApp/src/images/", but not "MainApp/images/"