Well, what you would do is create seperate bitmaps for all objects. For instance if you're using a 640x480 resolution, then make a 640x480 bitmap of you're background. Then you could make maybe a 80x80 bitmap for your character. If the character does not fill the whole space of the bitmap then fill the rest of the space with hot pink (255 R, 0 G, 255 B). Then make your bridge the same way as the character (same method...doesn't have to be same size of course).
Here is the code part of it:
This will draw the background to the screen and then over the background it will draw the bridge. At the same spot as the bridge it will draw the character over the bridge. This is what you are looking for, right? Hope that helps. If you have anymore questions just ask.
blit(background, buffer, 0, 0, 0, 0, 640, 480);
blit(bridge, buffer, 0, 0, x, y, 640, 480);
draw_sprite(buffer, character, x, y);
blit(buffer, screen, 0, 0, 0, 0, 640, 480);