Hi @Rakesh
Sounds like you are creating defaults for images and falling back to default images in your theme.
Perhaps this would be better left within the theme, as in, use an if statement to check if an image value exists and if not, use the one in your theme.
To do what you wrote in the previous comment would require that you upload all default images to the WP library on theme activation. This would require a lot of code and PHP skills.
Cheers
E
