How do I add an image to blogs? May you give me few hints on how to enable adding 1 optional image to each blog entry?

I've enabled Blog and Image modules and registered users can create and comment on them.

What is the next step?
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!