I was always hopeful that Transmutr would create the billboards automatically based on what we are seeing in the Transmutr window. If one wanted to replace with a rendered version, they could do that after importing, but in most cases, I do not have a billboard of the model upon converting, so having to pick a file for one seems like a feature that isn’t very useful.
At the moment, I am simply taking a screen grab of the model and using that as a billboard image but it is not transparent which is a bummer. If Transmutr could just generate a transparent PNG of what is in the Transmutr viewport would be a really incredible feature.