When changing the icon for Mac agents on the System Tab -> Site Customisation -> Agent Icon page, you see an image saying "Custom Icon" rather than the icon that you uploaded.
The agent icon will be deployed to the agent and work as expected.
Most web browsers, certainly, IE, Chrome, and Firefox are not able to display TIF files natively. This means that there is no way to show the image, as the browser itself cannot display the TIF file image.
WORKAROUND / RESOLUTION
This is expected behavior - "Custom Icon" is displayed if a custom icon is used.