I appreciate your reply Midnight Blues - it was certainly intended to be more helpful than the previous poster - but in the meantime I have had the opportunity to do my own experimentation.
I must point out that the Nuvi 1490T to which I refer was purchased by me in the UK in July 2009 so it came pre-loaded with the UK/Western Europe maps.
In order to get these maps installed in the 1490T device, you firstly have to rename the downloaded, unlocked .img file "gmapsupp" (without the inverted commas).
When the unit is connected to a PC and the "Nuvi" folder has been opened, look for a folder named "Map" (without the commas) - if it is not present just create it.
I must emphasise that this folder does not, and should not, exist in the hidden "system" folders - these folders contain the pre-loaded basemaps that the unit was sold with and are not named "gmapsupp" files.
Copy and paste the re-named .img file to this folder and remove your device from your PC - if there are already any "gmapsupp" mapfiles in that folder then it would be advisable to remove them to a back-up to avoid any conflicts - you can always replace them later.
Hey Presto! - it all works fine.
Just make sure that when you next start your device you go to Tools/Settings/Map/Info and ensure that only your chosen maps are ticked.
Before any "more knowledgable" poster tries to tell me it can't work etc., I emphasise that this has worked 100% for me today.
The only qualification is that I am referring to a Garmin Nuvi 1490T satnav device - it's quite possible that this technique will not be appropriate for a different model.
I really hope this helps some members - I will post further when I've experimented with installing these maps onto a MicroSD card for the 1490T.