That's be because they don't have their own image. They use the Starbase image:
Code:
Name := Baseship
Short Name := Baseship
Description :=
Code := BA
Primary Bitmap Name := Baseship
Alternate Bitmap Name := Starbase
Vehicle Type := Ship
Tonnage := 1500
Cost Minerals := 1500
Cost Organics := 0
Cost Radioactives := 0
Engines Per Move := 1
Number of Tech Req := 1
Tech Area Req 1 := Ship Construction
Tech Level Req 1 := 9
(rest snipped)
check vehiclesize.txt and see for yourself, or just start a High Tech game.