There is a bug in the editor when using the image manager to insert an image after creating a new image.  This is recreated on the live demo site - http://demos.telerik.com/aspnet-ajax/editor/examples/overview/defaultcs.aspx

1. Open the image manager in the editor.
2. Click on an image editor after selecting an image.
3. Use the resizing tool to resize and save a new version of the image. 
4. Once the save is made, both images (the original and the newly created) are selected and when you click insert, both images will be inserted.