If AltImage is set on this VehicleType then the vehicle will use a different image when it launches its spawns. Examples include the Dreadnought, Destroyer, and V3 Rocket Launcher. The image is internally stored in the same memory buffer as the turret so a unit cannot have both a turret and an alternate image.
The AltImage filename is derived from the normal Image by suffixing it with WO ("without"). For example, the Destroyer (DEST.vxl) has an alternate image of DESTWO.vxl.
This will cause an Internal Error if set on a vehicle that does not have the ability to spawn objects.
Note
In addition to the .WO extension of Voxel files, I have found other similar extensions of Voxel files:%sW APC