Hi Suzanna,
Thank you for contacting Syncfusion support.
We have an Image and File browser tool for inserting images in RTE. Please refer to the below given links
Through image browser you can upload local images into file explorer using the upload icon as shown below
From here you can insert the required images inside RTE.
Currently, we have provided tag helper support for Image and File Browser properties in ASP.Net Core internally and this will be included in our upcoming Volume 1, 2018 release which will be rolled out by the month of February. We will let you know once it has been rolled out.
Regards,
Keerthana.