Color Depth, GetPixel, and Bit Rot

[Zero]

I'm using DirectX 7 for this, since I have at least a vauge notion of what I'm doing with it...

What determines the color depth of a surface? If I load a 24 bit per pixel image into a surface but DirectX is chugging along at 16bpp, will the image be 'scaled back' to 16 bits? Will GetPixel (GetLockedPixel, whatever) return a 16 bit or 24 bit value?

~Zero the Inestimable