Currently the only format available for export is .png. Until PowerVTT has online play available and solidified, I will be using the builder to export files for use with the online competitor.
Now I don’t understand the technical reasons, but it has been reported that large .png can cause performance issues when viewing via a browser. This is less of an issue for individual smaller assets (i.e. tokens that require transparency) but when building large maps, having the option to export as a .jpg would be preferable.