I think you need to move your loadimage statement to after the SetMaskColor line like so
by the way, I see that you've grouped your canvas to pnl_canvas. I can't see where this was defined
SetGraphics CanvasGraphics(canvas_Buttons) SetBlend ALPHABLEND SetClsColor 0,0,0 SetMaskColor 255, 0, 255 Global gOpen:TImage = LoadImage("incbin::images/StringOpen.png",MASKEDIMAGE|MIPMAPPEDIMAGE)
by the way, I see that you've grouped your canvas to pnl_canvas. I can't see where this was defined