Auto-size routines (see AutoSize) may treat the image like always having at least these minimal sizes. Even if the Image is empty (Nil). This is useful when you have a row of buttons (typical for toolbar), and you want them to have the same height, and their captions to be displayed at the same level, regardless of their images sizes.
When AutoSize is True (the default) then Width/Height are automatically adjusted when you change the Caption and Image. They take into account Caption width/height with current font, Image width/height, and add some margin to make it look good.
To be more precise, Width is adjusted only when AutoSize and AutoSizeWidth. And Height is adjusted only when AutoSize and AutoSizeHeight. This way you can turn off auto-sizing in only one dimension if you want (and when you don't need such flexibility, leave AutoSizeWidth = AutoSizeHeight = True and control both by simple AutoSize).
Note that this adjustment happens only when OpenGL context is initialized (because only then we actually know the font used). So don't depend on Width/Height values calculated correctly before OpenGL context is ready.
When auto-size is in effect, these properties may force a minimal width/height of the button. This is useful if you want to use auto-size (to make sure that the content fits inside), but you want to force filling some space.
Can the button be permanently pressed. Good for making a button behave like a checkbox, that is indicate a boolean state. When Toggle is True, you can set the Pressed property, and the clicks are visualized a little differently.
Is the button pressed down. If Toggle is True, you can read and write this property to set the pressed state.
When not Toggle, this property isn't really useful to you. The pressed state is automatically managed then to visualize user clicks. In this case, you can read this property, but you cannot reliably set it.
If the image has alpha channel, should we render with alpha test (simple yes/no transparency) or alpha blending (smootly mix with background using full transparency).