We really need more categories for truck descriptions, all we have is "Simple truck", "trailer-truck", "tow-truck" and "Fire Engine". What about garbage trucks or flatbed trucks or tankers?

"simple truck" is a temporary name, because I still could not find a translation of the french "porteur" category, i.e. trucks like this:
http://www.obrist.ch/images/Camion-Gay.jpg
http://www.senacyt.gob.pa/g_metrologia/cenamep/artes/camion.png
http://www.marbriella.it/comitato/foto/camion.jpg
...

The garbage trucks can go in that category, though we could also create a special group for them.

Tankers and flatbed go either in this category or in "trailer truck", depending if they have a trailer or not.
A trailer truck with a tanker or flatbed is usually the same truck as another trailer truck, so for these there is no need of a special category. And so I find strange to make lots of category for the non-trailer trucks.

Your example pics are of lorries not trucks

But we still have catagories for Ambulances, which are based on trucks.

Well, as far as I know the US "truck" name is for the UK "Lorry" name.
That's why I used "truck", as on IMCDb all names were using US names (e.g. "Sedan" rather than "Saloon").
And currently the "trucks" are splitted between light trucks (cf the categories pickup, ambulances, chassis-cabin, etc.) listed with cars, and medium/heavy trucks listed in "Trucks".
This separation was made because it is the usual split for driving license and other legal stuff in European countries (at least France and Belgium). I do not know how it is in UK, and I know that it is different in US.

I'm not sure it's a direct substitute. I think US "truck" is a lot more "wooly"

For UK driving licence types see http://www.dvla.gov.uk/drivers/drivers.htm