Can you use SDL_CreatRGBSurface for full color Images,
like mountain scenery?
Of course you can 
just use the correct depth (32 bit in your case) to
do it.
See the SDL Documentation, there you find out, how to set the color
depth
Might be a silly question, I just imagine color loss
when I think RGB.
RGB would definitly be true color, cos you have 8bit=1byte presenting
the red channel, 8bit for the green and 8 bit for the blue channel, so
it will be 3byte or 32bit what makes it be a true color surface.
In your case (mountain-scenery) I would check, if I couldn’t represent
that in 16-bit mode, what should be enough for that 32-bit sounds a
little bit like a waste for this, even if you use a photorealistic
background.
Greetings,
Sascha