Currently the boot images PMX.IMG and BARE.IMG are created by a batch file that uses DOS's copy command. No utility to do this any other way has been created yet. See the file MAKEIMGE.BAT for an example.
There's 2 batch files called MAKE144.BAT and MAKE12.BAT that create boot floppies from the default boot image (PMX.IMG). Make sure a fomatted floppy is in drive A: before starting the batch file.