Badly kept secret here that I'm also Jonathan Roberts. I should do a tute on that isometric style. I don't think there are any tutes for iso maps here yet. I'd have a look at recent threads by Djekspek or by DevinNight. They've been doing quite a lot in similar styles for a while.

If you're after a true iso map then I'd suggest drawing out the floorplan in plan view to a grid first, then rotating it 45 degrees and shrinking the vertical axis by 57.7% Do the same for the grid. That should give you the basis, then freehand in the details and colour.

As for the gimp files - save it out as a jpg and then open the jpg in gimp. Go to Image->Image size (I think?) and reduce the size to around 2000px or so. Then save as to save a downsized copy. When you use the attachment manager on the forum it will create a thumbnail for you, so no need to worry about that. Throw the large file up and people can see all your work in its full glory!