I know that you have done some graphics
There is 2 ways of doing that one is to make your building in to a tile (set) and import it on to a map. there is a folder in IETME that has tilesets and you can make your own directory too. you can use DTLCEP or IETME I recommend the latter espcialy for a beginner.
The other way is to make a new map using a graphics program (PH, PSP, or the GIMP). If you make your own map you have to make sure that the over all size is a multiple of 64, meaning that the height is a multiple of 64 and that the width is a multiple of 64. Not to exceeded a 70x80 map.
As long as the angle looks close to what BG uses it should be fine.
did this answer your question? it is not very difficult, just keep asking questions if you have a problem.