We will have the IE6 issue of Set Image Properties dialog fixed with the next release.
In the meantime, please, find attached a temporary solution - SetImageProperties.ascx. To put it to work, you have to use it as an external dialog, i.e. you have to:
1. Create a directory named, for example "Dialogs" in your project;
2. Copy SetImageProperties.ascx file in it;
3. Set the ExternalDialogsPath="~/Dialogs/"
property in the server tag of RadEditor.
Please, let me know if you experience further difficulties with this.
the Telerik team