Centering the Bitmap for both for HD and SD?

You can probably check the rodeviceinfo for the current screen resolution or type setting.
Any of these would suffice --

Function GetDisplayType()
	Return CreateObject("roDeviceInfo").GetDisplayType()
	'4x3 standard
	'16x9 anamorphic
	'HDTV
End Function

Function GetDisplayMode()
	Return CreateObject("roDeviceInfo").GetDisplayMode()
	'480i
	'720p
End Function

Function GetDisplayAspectRatio()
	Return CreateObject("roDeviceInfo").GetDisplayAspectRatio()
	'4x3
	'16x9
End Function

Function GetDisplaySize()
	Return CreateObject("roDeviceInfo").GetDisplaySize()
	'Associative Array with values W for width and h for height
	'{w:1080 h:720}
End Function

You would put your hook to load the graphic you want based on the result of any of these functions.