Hi Praveen,
I’m not a scripter, but it seems, that only the full path with ":" instead of "/" is honored on a Mac?!
Steps to reproduce:
1. Import your image on a page in InDesign and select it
2. app.selection[0].graphics[0].itemLink.filePath; // gives you the full path of your image
3. myDoc.epubExportPreferences.coverImageFile = 'MBook:Users:mbadmin:Desktop:E:Cover.jpg'; // works for me
best
Kai