I actually use the GIMP to make most of the preview images you see They're 512x512 single-layer DDS images.
Start positions are defined in LobbyOverlay.dds which is also a 512x512 image. Though they're generated automatically on our internal build; not sure when that's going out to public if it isn't already.
Also, the start position order is defined by the order you put the Nexuses on the map. You can do a hacky manual assignment by noting which Nexuses are assigned to which start number in-game, then dragging them around to different positions in the editor.