To set or change a vehicles icon, click on Vehicles (1) on the left-hand side of the screen or alternatively, click Maintenance (2) then Vehicles (3):
Select the vehicle you wish to change or set the vehicle icon for.
From the tabs on the top of the edit vehicle screen, click on Information (1):
Under the header Vehicle Information, select a Vehicle Image (1) from the Vehicle Image drop-down:
Click Save and Close to update the vehicle icon.