From Adventure Game Studio | Wiki
About alpha-channel sprites for GUIs:
- GUI controls do NOT use alpha channel, just a transparency color. There is a workaround, see below.
- GUI backgrounds CAN use an alpha channel.
- If your GUI background has an alpha-channel image, then its transparency will be applied to any control on that GUI, i.e. the control becomes invisible. Unless the GUI control, too, has an alpha-channel image applied, in which case the alpha-channel image will be displayed!
- GUI Buttons with a transparent color will respond to clicks over the entire defined rectangle (no pixel-perfect detection).