Textbox.io for ASP.net embeds local image in the content, then extracts them on the server. Uploading images on insertion is not supported.
See Image uploads in the Textbox.io for ASP.net documentation
See Handling Local Images in the Textbox.io documentation.
Editor Types - Classic vs Inline
Only classic mode is supported.
See Editor types - Classic vs Inline in the Textbox.io documentation.
This framework has not been tested and is not officially supported.