RE: Counterfeiting.
Of course Imperial currency can be counterfeited. Forgery at the very least is canonical, and counterfeiting appears in some quasi-canon. The trick is to make it difficult and punish transgressors. The main way to makle counterfeiting difficult is to use production processes that are expensive. If you can afford the 5 million credit printing press required to counterfeit, why bother to counterfeit in the first place?
Imperial money needs to be very difficult to copy, but also usable at every tech level. Electronic money won't get you far on a low tech world.
Commodity based currency sounds like a great idea until you realize that commodity values are going to vary widely between worlds. A gold coin won't have much value on a world where gold is common. And shipping commodity currency between worlds is going to be a pain - particularly in large sums.
Imperial currency is probably fiat money, with distribution controlled by Imperial edict, probably through certain MegaCorp banks in much the ame way that the Federal Reserve (a private corporation) controls the supply of money in the US. There is probably a local branch on each world of any reasonable size, which controls the amount of currency in circulation - unless you use Imperial currency only for inter-world transaction, and allow local currency on each planet. In the case of the latter, you will have to deal with exchange and exchange rates.
When you start to delve into this, you find the economics of Traveller aren't very well thought out - but then again it isn't a game by and for economists.
INTU, I assume that the Imperial credit, much like the dollar of old, is welcomed everywhere as 'real money'. It is often preferred to the local stuff, and any trader or black-marketeer will know the price in credits of anything bought or sold in the local market.
Most planets won't even bother with printing their own money, but just adopt the Imperial Credit. In this case, the roll of the central banks will be critical, with the availability of credits on world strictly controlled so that the Credit has similar purchase power throughout the Imperium to prevent speculation by shipping tons of imperial notes between planets.
As far as the currency itself, I did a write up a while ago at the request of my players. It can be found here:
http://travellercentral.com/rules/money.html