Doesn't have to be square, some kind of rectangle anyway. Check out the VT forums on these boards, or look earlier in this thread, lots of questions were answered here.

JPG or PNG are the preferred formats for VT apps. Resolution here is 50 - 100 ppi and consider all maps will be viewed on PC moniters not printed maps.

The tiles in my entry are huge, 24" x 36" but that's just the way I chose. Many prefer tiles no larger than 2000 x 2000 pixels. Also tiles shouldn't exceed 1 MB in file size - at least those are the needs for the sponsors VT app, RPGTonight.

I'm sure a better expert may come along and help explain all this to you.

GP