I assume you're using an Image/File attribute?

If so, go to Dashboard>Themes>Page Types

Click on Defaults for your Blog Page type.

From the Page Defaults, you can apply Page Properties that are set on every new page of that type.

Just add your default image here and save it.

If you have a bunch of blog posts already created and want to apply the default header image en masse, go to Dashboard>Page Search

Search for all pages of your Blog Page Type.

Select them all with the check box.

From there, in the "With Selected" dropdown, choose "Edit Properties."

Here you can set your image attribute to every page at once. Just make sure you click the little pencil/paper icon to save before you close out of that window.