[Holger]

A normal TIFF-file should be opened with a standard program in Windows when double-clicking it.

Are you saying the sites are using TIFF as webgrahics in the websites? Or did I understand wrong?
If those images cannot be opened, then try to rename them to .jpg or .png or .gif. Files contain a "header" which describes what they are. But it may confuse browsers if the extension is .tif and the header says .jpg