As I said before, the key has no specific format. It can be any length - 10, 20, 50, etc. characters. Naturally, the longer the key, the more secure it is.
As for the Image Properties dialogs, I think the problem there could be coming from the dialog title, which is also part of the dialog URL. The dialog title is "Properties..." and some firewalls do not like the three consecutive dots. To verify if this is the case, try setting a different title for the dialog from the code file of the RadEditor page:
sender, EventArgs e)
where RadEditor is your editor instance.
All the best,
the Telerik team