How do I add an image to blogs?
Go to "Manage Fields" link for the Blog content type (/admin/structure/types/manage/blog/fields) and add a new field:
Name it whatever you want ("blog_picture" or something) and for the Field Type select "Image"
Click "save", pick a default image if you want and click "save field settings"
On the next screen do not tick "Required", make any other changes you want and click "save settings"
Done!