Need to replace one cursor of a set; have already created the PNG for the cursor

I have created a replacement for xterm cursor that will allow me to see the cursor (Windows, incidentally, calls the xterm cursor the "i-beam", which is rather more descriptive) much better. Problem is, I've yet to be able to correctly convert the PNG to the xcursor format. Anyone know what I can do here? OS: Linux Mint 17.3 (Ubuntu-based; understands Debian packages)