Fonts in Greenfoot

What do you mean by "changing the default Font"? You're correct that you can't change the default font, i.e. the font that you get when you use the default Font constructor. You can also use an alternative constructor to specify a font name, though it will only find system fonts (i.e. you can't make it use a TTF font included in your scenario). You can use a font created in this way to draw on an image. However, the GreenfootImage constructor which creates an image from text always uses the default font. I hope that clears it up.