To display image field to blog post which will select from existing images in all image libraries.
1. Add new custom field for blogs
2. Set its type to short text
3. Set "Interface widget for entering data" to Custom
4. In the custom textbox enter: Telerik.Sitefinity.Web.UI.Fields.ImageField
5. Save the custom field and test it in blog post item.
Then go to the widget template for blog posts accessible when editing blog posts widget placed on a page and in the template you will be able to see the name you chose for the custom field created in Step 2. click it and it will add the code that will display the image at the frontend. for reference you can review the video to this blog post.
To prepopulate the image field on blog create will be more complicated because both a custom field and blogs post template and code file will have to be modified to allow this.
Here is a form post describing similar process.