Firstly, welcome aboard...

To similate a re-sizable bmap you are best off, in my opinion, to use a panel.

First, define yourself a "splash screen" using a panel, and assign your bmap to it.
DONT assign the panel an x_size or y_size, and it will inherit the size of the bmap.
Then whenever the screen resolution is changed, or when making the panel visible,
you need to recalculate the panels scale_x and scale_y values according to the required result...

EG : (assumes panel is named 'splash')
splash.pos_x = splash.pos_y = 0;
splash.scale_x = screen_size.x / splash.bmap.width;
splash.scale_y = screen_size.y / splash.bmap.height;