That's the whole point FtxCommando. You don't know if something is amphibious without clicking on it and finding the name of the unit. There's nothing to give you this information in the unit icon. It's just a regular diamond like all other tracked/hover vehicles.
There's also no information on what kind of experimental something is. You need to click on it to find out if it's a Megalith or a Fatboy.
If you're worried about not being able to tell it's a T4, that's why it could be inside a hexagon, very nice and easy. It will tell you it's a T4 and what T4 it is. Nothing is lost, only gained.
Eventually I'd like to have a different icon shape for buildings, air, land and sea experimentals but we really don't have a lot of shapes left that we can differentiate in such a small icon. We can use cross shapes but the problem with those is that the space to put identifiers in that kind of shape is very small and the icon becomes hard to read. This is mainly why extra symbols is an easier solution.
The problem with making individual image icons for every T4 is that the icons are between 12x12 and 20x20 in size. Something like a monkeylord is perfect for this but GC, Ythotha and Commander all look very similar in this size and differentiating between them reliably is very, very hard. Also what do we draw for a fatboy, it's a box. Do we draw a box? We can't, box is for buildings. Or maybe we do, because it's a factory? Do we draw a shield dome? We can't because domes are for ships. So maybe a toy car? That could work, but the image association isn't really any better than a random symbol then. A lot of the experimentals are the same, they're just geographic shapes without identifying features. This is why unique, easily associated images is problematic.
This also shows the possible cross shapes, as you can see, less than ideal. The pointy cross is very close to the diamond already in use for tracked vehicles, the short arm cross is too close to both the hexagon and the diamond while the regular cross doesn't have enough space. The diagonal cross is the best but still too small to add a symbol in there.
The letters and regular markings inside the hexagon work well though. They're clear and easily distinguishable.