I think there is a bit of confusion for you here, Trashman. Both the weapon selection icon (thing that you pick up and put into the weapon bank on the fighter) and the technical animation (picture of it spinning on the green grid) are Freespace .ani files.
As an example, the Prometheus-S cannon uses the file IconPromS.ani for its selection icon ANI and Tech_GTW-5_Prometheus_S.ani for the tech animation. Weapon selection icons have four frames in them with dimensions of 56x24 pixels, and are in 256 colour mode. Tech ANIs for weapons have 139 frames, dimensions of 440x200 (for low resolution play, anyway), and again in 256 colours.
As a piece of advice, when you make the icon for your new weapon, try to keep the colours similar to those in the original ones. It looks a lot better.