I am not sure whats going on.But may be I am suspecting the format of the map(.png).Can you try JPEG and see how things work.May be then we will go from there.
Here are few other things to verify.
-Files used for linked backgrounds must be continuously accessible by URL reference.
-Files used for static backgrounds must be available within the local file system.
-Background images you supply display at their full size in the Orion Web Console, so consider their dimensions. You may rescale images within the application, but images displayed at full scale provide optimal quality.
-In determining map size and resolution, consider web page layouts and potential display screen resolutions.
-Example background images are in the Network Atlas Backgrounds folder located in your default shared documents folder. Clicking Background Image always starts you in this background images folder.