the type of map you describe is what i refer to as a diagonal or diamond iso map, and it is the type used in sim city, aoe, etc.

the x and y axes go in diagonal directions, which is probably what''s giving you trouble.

make the top corner of the map tile (0,0) -- actually, any corner will do.

pick a direction for x to increase (either southeast or southwest, i usually pick southeast), and the other direction is the direction for y.

now, assuming you are using 64x32 tiles, to move in the x direction you add +32,+16, and to move in the y direction you add -32,+16.

so, to render your tiles...

int tilex;
int tiley;
for(int y=0;y<mapheight;y++)
{
for(int x=0;x<mapwidth;x++)
{
tilex=x0+x*32-y*32;//x0,y0 is a reference point for the upperleft corner of tile (0,0)
tiley=y0+y*16+x*16;
//use tilex and tiley to blit your tile
}
}

this will give you the map shape you are looking for.

fyi-the type of map you wound up with (the jagged rectangle) is what i refer to as a staggered map. it is used in games like civilization.