To add graphics to buttons you first need to add them to the Image Library - it can be accessed via the menu bar - Windows menu dropdown. There is a section about this in Chapter 2 of the Language Guide PDF.
If i need a button with graphics I usually use a Canvas control because they have many more options and events than buttons do. For instance, when your graphic is loaded into the canvas it is possible to draw into the canvas and modify your graphic, and then even save the changed graphics.
There are demo projects featuring these and many other techniques in the Projects that come with the HyperNext package or listed on the Projects page of our website.
The Features project has code for loading an image into a canavas, copying and scaling.