You can now get the Plugin Developer API using THeadsAPI.getInstance(), you can find the 2 main methods which share a common syntax since 1 of them is just the overload of the actually coded one.

in this case the getHeadItem(String location, boolean includeDefaultUrl) does this (simple wiki)

if you specify the includeDefaultUrl to be true, the old education system will be used (WARNING: It doesn't work and its outdated) instead you should put this to false and give it the URL yourself as a String.

https://jitpack.io/#nicecraftz/TransparentHeads