How we can set an image as default value in assets field
That's not possible, but the way I handle this is to have a Global called "Missing Image Fallback" inside a "Generic" Global field set, and then when I need to reference an image I use the following twig:

{% set image = entry.myImageField.first ?? generic.missingImageFallback.first %}


That will assign the first myImageField asset to 'image' if it's been filled in, but fall back to the generic global field if not.