The issue is not related to the Image manager of RadEditor, but to the ability of the browser to display TIFF files. The browsers themselves cannot read and display TIFF files - you will need additional ActiveX component or browser plug-in (for Mozilla, Netscape, etc.).
For example if you load a tiff image in the browser, you will see that Internet Explorer just opens a download dialog box and cannot display the file.
the Telerik team