I did not manage to reproduce the problem. I tested it with Sitefinity 6.1.4600 and our latest release Sitefinity 6.1.4700. I recorded a short video with my steps. Here is a link to the video:
I added an Image field to blog posts, then I created and edited a blog post and selected some images.
Would you give us more details about your project? Which version of Sitefinity are you using? Do you have any languages added? What are the other custom fields of your blog posts? Do you reproduce this issue on any browser or is it reproducible only on a specific one?
Currently, we set "Telerik.Sitefinity.Web.UI.Fields.ImageField" type to a short textbox. This is why the value is stored in the database with a fully qualified URL path.
The following blog post might be useful to you:
Soon there will be an update of the thumbnail selector from this blog post, so you can check for updates.
I am attaching a fixed ImageSelector for 6.1 that you can try in your project.
You have to put the following code in the Global.asax file in order to register the field controls.
sender, EventArgs e)
sender, Telerik.Sitefinity.Data.ExecutedEventArgs e)
position: inherit !important;